RESTORATION AND MANAGEMENT OF LAKES AND RESERVOIRS - CHAPTER 13 potx

13 Water Level Drawdown13.1 INTRODUCTION Water level drawdown is an established, multipurpose reservoir and pond management procedure to control certain aquatic plants and fish populatio

13 Water Level Drawdown

13.1 INTRODUCTION

Water level drawdown is an established, multipurpose reservoir and pond management procedure

to control certain aquatic plants and fish populations, and possibly to produce a switch in alternative stable states (Chapter 9) It is less commonly used in lakes without an outlet control because siphoning or pumping (Chapter 7) is needed It provides opportunities to repair structures such as dams or docks, to remove or consolidate flocculent sediments, and to carry out dredging or sediment cover installation

This chapter emphasizes drawdown to reduce macrophyte biomass, and describes case studies from several North American climates Responses of 74 plant species to whole-year, winter, or summer drawdown are presented as a user guideline A discussion of its use in fish management

is included, and positive and negative factors of the procedure are summarized Drawdown can also be used to encourage regrowth of emergent species (Chapter 12) Reviews include Cooke (1980), Culver et al (1980), Ploskey (1983), and Leslie (1988)

13.2 METHODS

The primary mode of action of water level drawdown for macrophyte biomass management is exposure of plants, especially root systems, to dry and freezing, or dry and hot conditions for a period sufficient to kill the plants and their reproductive structures Winter drawdowns are more successful than summer, although the number of reported summer drawdowns is too small for adequate evaluation The advantages of winter drawdown, in addition to effectiveness on some target plants, are: (1) there will be no invasion of moist lake soils by semi-terrestrial plants, (2) there will be no proliferation of aquatic emergents, and (3) there will be less interference with recreation Also, runoff is often highest in the spring, so refill should occur The decision to employ

a summer or a winter drawdown to control plants depends upon target species susceptibility, uses

of the reservoir, and other management objectives

Aquatic plants do not respond uniformly to drawdown Table 13.1 is a list of the responses of

74 species Some are unaffected or increase in biomass, while others are very susceptible Because

of this, accurate plant identification is required

Table 13.2 is a summary of responses of 19 common plants to drawdown Cutgrass and smartweed are among those that grow well in moist soils and shallow water, and will proliferate

in some drawdown situations This may be desirable when attempting to enhance a fishery, as explained in later paragraphs Alligator weed and hydrilla are serious nuisances in southern U.S waters and are rarely controlled by this procedure Milfoil and water hyacinth have been controlled

by winter drawdown, particularly Myriophyllum spicatum (Eurasian watermilfoil) This plant,

however, as shown by experience in Tennessee Valley Authority (TVA) reservoirs and in Oregon, withstands low temperatures if the plant remains moist or if the exposed hydrosoil is not frozen for several weeks Milfoil is also well adapted to rapid vegetative spread It may recolonize areas dominated by native plants prior to drawdown

In lakes with a mixture of species, exposure of littoral communities to dry and hot or to dry and cold conditions may eliminate or curtail one plant species and favor the development of a

TABLE 13.1

Responses of 74 Aquatic Plants to Water Level Drawdown

Species

Increased Decreased No Change

Alternanthera philoxeroides 10 9 15

31

Brasenia schreberi 1 13

26

Cabomba caroliniana 15 11 17

Cephalanthus occidentalis 15

Ceratophyllum demersum 28 20 14 1 13 21 15

9 11 16 32

Chara vulgaris 16 17 15 30 14

35

Cyperus spp. 10

Eichhornia crassipes 9 15 10 11

35

Eleocharis baldwinii 15 17

Eleocharis acicularis 13 1

20 33

Glyceria borealis 21

Hydrilla verticillata 3 18 36

Hydrochloa caroliniensis 10

Hydrotrida caroliniana 21

Leersia oryzoides 21 13

Lemna minor 28

Myriophyllum brasiliense 15 14

Myriophyllum exalbescens 2 30

Myriophyllum heterophyllum 26 15

Myriophyllum spicatum 4 5 30

24

25 33 35

11

Megalodonta beckii 1

6 21 24 33

Najas guadalupensis 10 9 17

14 16

Nuphar macrophyllum 9

21

15

Panicum sp. 10

Polygonum coccineum 21 8 1

Potamogeton americanus 21

Potamogeton amplifolius 20 1

2

35

Potamogeton diversifolius 1 15

19

21

Potamogeton natans 1 13

Potamogeton pectinatus 28 34 6

34

Potamogeton Richardsonii 21 1

Potamogeton Robbinsii 1

2 20

TABLE 13.1 (Continued)

Responses of 74 Aquatic Plants to Water Level Drawdown

Species

Increased Decreased No Change

resistant one Some susceptible plants such as milfoil, as noted above, are normally so successful that few other species coexist In these cases several years of winter water level drawdown, followed

by no drawdown for 1 to 2 years, may prevent establishment of resistant species by allowing other species to reestablish The drawdown cycle can then be repeated

Management of macrophyte biomass, and fishery enhancement, through systematic changes in water level, are not possible with every water body where water level can be regulated Hydropower storage and flood control reservoirs are most amenable to water level management The strong influence of flow and the limited storage capacity of main stem reservoirs limit their water level manipulations for management purposes (Ploskey et al., 1984) Other factors prevent or limit the use of water level drawdown for management, including water supply use, summer or winter

33

Potamogeton zosteriformis 19 1

2

Salix interior 21

Scirpus americanus 1

Scirpus californicus 10

Scirpus validus 21 29

Sparganium chlorocarpum 29

Spirodela polyrhiza 1

Typha latifolia 18 21 29 10 1

Utricularia purpurea 26

Utricularia vulgaris 1

Vallisneria Americana 1 10 2

Note: A, whole-year drawdown; W, winter drawdown; S, summer drawdown Numbers

refer to references given as sources below.

a Summer-fall drawdown.

Sources are from the Reference list as follows: 1 Beard, 1973; 2 Dunst and Nichols,

1979; 3 Fox et al., 1977; 4 Geiger, 1983; 5 Goldsby et al., 1978; 6 Gorman, 1979; 7.

Hall et al., 1946; 8 Harris and Marshall, 1963; 9 Hestand and Carter, 1975; 10 Holcomb

and Wegener, 1971; 11 Hulsey, 1958; 12 Jacoby et al., 1983; 13 Kadlec, 1962; 14.

Lantz et al., 1964; 15 Lantz, 1974; 16 Manning and Johnson, 1975; 17 Manning and

Sanders, 1975 (summer–fall drawdown); 18 Massarelli, 1984; 19 Nichols, 1974; 20.

Nichols, 1975a; 21 Nichols, 1975b; 22 Pierce et al., 1963; 23 Richardson, 1975; 24.

Siver et al., 1986; 25 Smith, 1971; 26 Tarver, 1980; 27 Tazik et al., 1982; 28 van der

Valk and Davis, 1978; 29 van der Valk and Davis, 1980; 30 Wile and Hitchin, 1977;

31 Williams et al., 1982; 32 Godshalk and Barko, 1988; 33 Crosson, 1990; 34 Van

Wijck and DeGroot, 1993; 35 Wagner and Falter, 2002; 36 Poovey and Kay, 1996.

TABLE 13.1 (Continued)

Responses of 74 Aquatic Plants to Water Level Drawdown

Species

Increased Decreased No Change

recreation, shoreline development such as parks or homes, the need to maintain water levels for downstream low-flow augmentation, and dam design that will not allow sufficient water release (Culver et al., 1980) Also, undesirable effects on non-target littoral zone or wetland species could prevent the use of this technique A permit to discharge enough water to expose the littoral area could be needed where wetland alteration or destruction could occur, or where discharge may affect downstream uses

TABLE 13.2

Summary of Responses of 19 Aquatic Plants to Water Level Drawdown

Species That Usually Increase

1 Alternanthera philoxeroides (alligator weed): Annual (Holcomb and Wegener, 1971), winter (Hestand and Carter, 1975),

summer (Lantz, 1974)

2 Hydrilla verticillata (hydrilla): Winter (Fox et al., 1977; Hestand and Carter, 1975)

3 Leersia oryzoides (cutgrass): Winter (Nichols, 1975b), summer (Kadlec, 1962)

4 Najas flexilis (bushy pondweed): Annual (van der Valk and Davis, 1978), winter (Beard, 1973; Crosson, 1990; Gorman,

1979; Nichols, 1975b; Siver et al 1986), summer (Kadlec, 1962)

5 Polygonum coccineum (smartweed); Winter (Nichols, 1975b), summer (Harris and Marshall, 1963 Beard, (1973) reported

a decrease in this species in a winter drawdown

6 Potamogeton epihydrus (leafy pondweed): Winter (Nichols, 1974, 1975b) Beard (1973) reported no change in this

species in a winter drawdown

7 Scirpus validus (softstem bulrush): Winter (Nichols, 1975b), summer (van der Valk and Davis, 1980)

Species That Usually Decrease

1 Brasenia schreberi (water shield): Winter (Beard, 1973; Hulsey, 1958; Richardson, 1975), summer Kadlec, 1962; Lantz

et al., 1964; Lantz, 1974; Tarver, 1980)

2 Cabomba caroliniana (fanwort): Winter (Hulsey, 1958; Richardson, 1975), summer (Manning and Sanders, 1975; Tarver,

1980)

3 Ceratophyllum demersum (coontail): Annual (Lantz et al., 1964), winter (Beard, 1973; Dunst and Nichols, 1979; Godshalk

and Barko, 1988; Hestand and Carter, 1975; Hulsey, 1958; Manning and Johnson, 1975), summer (Kadlec, 1962; Manning and Sanders, 1975) Increases or no change in this species were reported by Lantz, 1974; Nichols, 1975a, b; and van der Valk and Davis, 1978

4 Egeria densa (Brazilian elodea): Winter (Hestand and Carter, 1975; Manning and Johnson, 1975), summer (Jacoby et

al., 1983; Manning and Sanders, 1975)

5 Myriophyllum spp (milfoil): Winter (Beard, 1973; Crosson, 1990; Dunst and Nichols, 1979; Goldsby et al., 1978; Hulsey,

1958; Smith, 1971; Siver et al 1986), summer (Lantz, 1974; Tarver, 1980; Van Wijck and DeGroot, 1993) Increases and no change in milfoil have occasionally been reported; see Table 13.1 for species and references

6 Najas guadalupensis (southern naiad): Annual (Holcomb and Wegener, 1971), winter (Hestand and Carter, 1975; Lantz

et al., 1964; Manning and Johnson, 1975) Manning and Sanders (1975) reported no change in this species in a summer–fall drawdown

7 Nuphar spp (yellow water lily): Winter (Beard, 1973; Nichols, 1975a, b; Pierce et al., 1963), summer (Tarver, 1980)

Increases and no change in Nuphar have occasionally been reported; see Table 13.1 for species and references

8 Nymphaea odorata (water lily): Summer (Lantz et al., 1964; Lantz, 1974) Jacoby et al (1983) reported no change in

this species in a summer drawdown; Tarver (1980) reported an increase in a summer drawdown

9 Potamogeton robbinsii (Robbins’s pondweed): Winter (Beard, 1973; Crosson, 1990; Dunst and Nichols, 1979; Nichols,

1975a)

Species That Do Not Change, Or Whose Response Is Variable

1 Eichhornia crassipes (water hyacinth): Hestand and Carter (1975), Holcomb and Wegener (1971), Hulsey (1958), Lantz

(1974), Richardson (1975)

2 Elodea canadensis (elodea): Beard (1973), Dunst and Nichols (1979), Gorman (1979), Nichols (1975a, b), Wile and

Hitchin (1977)

3 Typha latifolia (cattail): Beard (1973), Holcomb and Wegener (1971), Nichols (1975b), van der Valk and Davis (1980)

13.3 POSITIVE AND NEGATIVE FACTORS OF WATER LEVEL

DRAWDOWN

Control of susceptible nuisance plants and fish management are two of the several ways that drawdown can be used to improve or restore lakes Ideally, if this procedure is to be implemented for plant control, the possibility of carrying out every other lake improvement procedure that drawdown makes possible should be considered

Grass carp and herbicide applications are effective for managing nuisance macrophytes in certain circumstances (Chapters 16 and 17) Water level drawdown can reduce the amount of grass carp needed, or improve their effectiveness (Stocker and Hagstrom, 1986), and provides opportunity for pelletized herbicide applications (Westerdahl and Getsinger, 1988)

Loose, flocculent sediments are common in eutrophic systems and represent a significant source

of turbidity, discomfort to swimmers, and a source of nutrients to the water column Drawdown is effective in consolidating some types of lake sediments The effects of drying on muck-type (organic and nutrient-rich, high in water content), flocculent (poorly defined sediment-water interface), and peat-type (fibrous, organic, low water content) sediments from Lake Apopka, Florida were examined

in the laboratory Muck-type sediments consolidated 40–50% after exposure to rain and sun for

170 days Peat consolidated about 7% under identical conditions (Fox et al., 1977) The 40–50% water loss may be sufficient to make the sediments firm to walk on, and consolidated sediments appear to remain firm after reflooding (Kadlec, 1962), although groundwater seepage might prevent these changes In some lakes, there is only a slight consolidation of sediments after a summer drawdown (e.g., Long Lake, Washington ; Jacoby et al., 1982)

Sediment removal could be combined with sediment consolidation to bring about deepening

of selected areas A bulldozer could be used for sediment removal instead of expensive hydraulic dredges, assuming sediments can support heavy equipment (Chapter 20) Since consolidated sed-iments have lower water content, and little water is removed with them during bulldozer operation, runoff from disposal sites is minimal and land reuse at the disposal site could be immediate An extreme drawdown of Lake Tohopekaliga, Florida was followed by a sediment removal of 165,000

m3 in 1987, 340,000 m3 in 1991, and 3,000,000 m3 in 2002 The 2002 dredging was projected to remove 120 t of P and 2,500 t of N (Williams, 2001) When sediments are exposed, debris can be removed and artificial reefs for anglers can be constructed

Loose flocculent sediments can inhibit growth of desirable macrophytes and prevent fish spawning Some of these sediments can be removed via lake outflow during a drawdown, although there may be downstream impacts At Newnan’s Lake, Florida, summer drawdown scoured the lake’s bottom, removing 270 kg P and 59,000 kg of flocculent sediments, and produced some sediment compaction (Gottgens and Crisman, 1991)

Water level management is an important part of the restoration of lake or reservoir “fringe” wetlands (Levine and Willard, 1989), and fluctuating water levels are essential to maintaining the vegetation supporting a waterfowl community (Kadlec, 1962) A Michigan waterfowl reservoir was drawn down in summer to stimulate the growth of plants attractive to ducks Emergent plants such

as Typha (cattail) and Scirpus (bulrush) prefer bare mudflats as a seedbed, a condition not met in

stable water level systems; drawdown provided conditions for the germination of their seeds (Kadlec, 1962) Up to 20,000 seeds per square meter were found in the upper 5 cm of exposed sediments in an Iowa marsh, a seed bank that should allow establishment of a community of emergent and annual species (van der Valk and Davis, 1978)

Drawdown presents other possibilities for lake improvement Sediment covers are more easily and cheaply installed on dry, consolidated sediments than by the use of SCUBA (Chapter 15) Repair or construction of docks, placement of riprap on banks, maintenance of dams, and removal

of litter can be carried out effectively after drawdown Finally, this procedure has the lowest cost

of any macrophyte management method unless pumps are required to lower the water level (Dierberg and Williams, 1989)

Partial water level drawdown could be used to re-establish rooted macrophytes in order to stabilize sediments This has been proposed for Lake Okeechobee, Florida where transparency has fallen sharply in some areas, possibly from migration of bottom mud toward the shore when water levels exceed 4.6 m Lowering water level by 1.0 m may reduce sediment transport, clarify the water, and promote macrophyte establishment (Havens and James, 1999)

Algal blooms have occurred after reflooding of dried and/or frozen lake sediments (Hulsey, 1958; Beard, 1973), suggesting that drawdown may be a factor in switching a lake from a clear water, macrophyte-dominated condition to an algae-dominated, turbid condition (Chapter 9) Factors causing blooms may be P release from reflooded sediments, along with fish control of algae grazers Total P concentration in the top layer of drying, highly organic marsh sediments increased, and decreased in bottom layers (as deep as 40 cm), as dessication proceeded by an upward flux of water to the sediment surface (DeGroot and Van Wijck, 1993) In Big Muskego Lake, Wisconsin, porewater ammonium N and SRP, and laboratory-based P release, increased following a dry-ing/freezing drawdown (James et al., 2001) Dried sediments from a eutrophic reservoir had significantly lower affinity for P than continuously wet sediments, perhaps because redox cycling

of Fe–P species stops when sediments are air-dried, leading to the formation of crystalline Fe molecules with low P sorption (Baldwin, 1996) These data suggest there may be significant P flux

to the water column at reflooding, especially from hydrosoils with high organic content (Watts, 2000) Phosphorus released from dried and/or frozen lake sediments may be P associated with bacteria cells (Sparling et al., 1985; Qui and McComb, 1995) Relatively brief periods of drying

or freezing may be all that is needed to produce P release at reflooding (Klotz and Linn, 2001) Field observations of P release following reflooding are uncommon and conflicting A summer

1979 drawdown (June–October) to control Egeria densa in Long Lake, Washington, a U.S region

of low summer precipitation, was successful in lowering the 1980 standing crop by 84% Nuphar polysephalum and Nymphaea odorata were unaffected, and macrophyte biomass recovered by 1981.

Water column total P and pH were lower, and dense cyanobacteria blooms were absent in summer

1980 In the months of reflooding following the 1979 drawdown, there was no increase in water column P (Jacoby et al., 1982) In contrast, P increased after reflooding of Backus Lake, Michigan (Kadlec, 1962) Algal blooms are not always a consequence of drawdowns Drawdown of a

hypereutrophic, and previously regulated lake (Zeekoevlei, South Africa) led to large Daphnia and

clear water, even though nutrients increased significantly following reflooding Fish biomass was apparently lowered by washout, allowing large-bodied zooplankton and the clear water state to occur (Harding and Wright, 1999) More field observations about immediate water chemistry changes following reflooding are needed

Drawdown exposes wetlands adjacent to the lake and may have impacts to wetland biota Drawdown at Lake Bomoseen, Vermont, produced major effects on a wetland containing several threatened or endangered plant species Effects on invertebrates were also severe The elimination

of native plant species from exposed littoral areas may allow nuisance species from deep water, such as Eurasian watermilfoil, to invade the exposed areas (Crosson, 1990)

Failure to refill following drawdown is a potentially serious problem This may be from a failure to close the dam at the proper time, or to drought Reflooding should begin in late winter

so that lake users can be assured of access to the lake during recreation season, and other uses of the reservoir can occur

There is a potential for low DO and an associated fish kill during drawdown, particularly if incoming water is rich in nutrients and organic matter and the remaining pool is small in volume Once water level is down, there are few possibilities for aerating it Fish kills due to low DO have been a concern, but reports are contradictory Beard (1973) found no fish mortality despite a 70% winter drawdown in a eutrophic reservoir, and E.B Welch (personal communication) found that

DO in Long Lake, Washington, did not fall below 5 mg/L during a summer drawdown of 2 m (Zmax

= 3.5 m) Low DO (but no fish kill) occurred in Mondeaux Flowage, Wisconsin, during a winter drawdown (Nichols, 1975a) In contrast, a fish kill in Chicot Lake, Louisiana occurred during a

summer drawdown (Geagan, 1960), and Gaboury and Patalas (1984) observed a fish kill in a Manitoba lake following winter drawdown and a loss of DO DO problems can occur when summer drawdown causes turnover of a thermally stratified lake so that water low in oxygen is suddenly introduced into surface waters (Richardson, 1975) During a winter drawdown of an enriched Wisconsin reservoir, sediments became resuspended in the river-like areas of the upper reservoir These sediments were high in organic matter, were anaerobic, and contained significant H2S and reduced iron High chemical and biological oxygen demand extracted any remaining DO from the water This condition moved downstream, removing DO from the lower reaches of the reservoir

A delay of drawdown until mid-January and a release limit of 25% of reservoir volume were recommended to prevent future DO problems (Shaw, 1983) However, this might not provide sufficient exposure to cold for macrophyte control The possibility that drawdown will produce an oxygen depletion in the remaining pool should be assessed, and aeration or artificial circulation devices (Chapters 18 and 19) may be necessary

Drawdown may have severe consequences to the invertebrate community, which in turn could reduce fish productivity as well as species diversity of the benthic community Also, the release of large volumes of water can create flooding conditions downstream In addition, release of nutrient-rich and/or anaerobic water will be deleterious to stream biota A late fall water release would most likely be oxygenated with lower nutrient concentrations (assuming water release at fall overturn), and thus have lower impact on downstream biota

There can be safety concerns with a winter drawdown if inflows (e.g., a winter rain/snow melt) cause the ice cover on the remaining water to float This could create open water or thin ice near shore

13.4 CASE STUDIES

The object of water level drawdown for nuisance plant control is to expose plants to freezing-desiccation or to heat-freezing-desiccation, destroying the plant body and the rhizomes or roots Exposure

to heat or cold may also be detrimental to seeds, turions, and tubers In some regions (e.g., Louisiana), water level fluctuations have been a principal plant control method (Richardson, 1975), whereas in others (e.g., the U.S Pacific Northwest) climate extremes are usually too narrow to provide the necessary harsh conditions The following U.S case histories illustrate responses to drawdown in several climates

13.4.1 TENNESSEE VALLEY AUTHORITY (TVA) RESERVOIRS

Hall et al (1946) were among the first to describe flooding and dewatering effects on aquatic plants

Several woody species require dewatering for establishment, including black willow (Salix nigra), buttonball (Cephalanthus occidentalis), green ash, (Fraxinus lanceolata), tupelo gum (Nyssa aquat-ica), and bald cypress (Taxodium distichum) Each of these must be dewatered to become

estab-lished Similarly, herbaceous weeds will not develop on sites that remain inundated until June Reflooding of woody and herbaceous plants, as discussed later, is an important technique to enhance development of populations of fish food organisms

Alligator weed (Alternanthera philoxeroides) is a nuisance in some TVA reservoirs Subfreezing

temperatures are lethal to above-ground parts in this mid-latitude, milder climate region of the U.S., but below-ground roots show little or no injury and overwintering fragments recolonize sites after

spring reflooding The water primrose (Jussiaea diffusa) forms floating mats and is destroyed by

dewatering and freezing In the case of the primrose, as well as two other nuisance plants

(blad-derwort, Utricularia biffa, and milfoil, Myriophyllum scabratum), plants may survive if the soil

remains moist during a winter drawdown (Hall et al., 1946)

The TVA reservoirs have been infested with Eurasian watermilfoil (M spicatum) It is

partic-ularly troublesome in reservoirs with differences of only 0.6 to 1.0 m between minimum and

maximum water levels (Goldsby et al., 1978) Although the herbicide 2,4-D was used extensively, drawdown was the most effective control method along shorelines where herbicide dilution occurred (Chapter 16) A 1.8-m (6-ft) winter drawdown at Watts Bar and Chickamauga Reservoirs killed all milfoil plants on well-drained shorelines In some areas, landforms of milfoil developed that later reverted to the aquatic form when inundated (Smith, 1971)

Eurasian watermilfoil in Melton Hill Reservoir, Tennessee was managed with 2,4-D and winter drawdown from December to mid-February (1971 to 1972) The area colonized in 1972 was less than 1971, especially in shallow water, and deeper water plants did not increase in biomass From

1973 to 1976, 2,4-D was used, but costs increased steadily The herbicide brought about a 68% reduction in areal coverage in 1973 compared to the 1972 coverage after drawdown, but reinfestation was rapid unless herbicides were reapplied or drawdown was used Semi-monthly winter drawdowns were effective in destroying root crowns of plants exposed to freezing, but even harsh winters did not reduce infestations unless the hydrosoil was completely dewatered A combination of mainte-nance 2,4-D applications and high frequency, short duration winter drawdowns was most effective

and economical for control of M spicatum in Melton Hill Reservoir (Goldsby et al., 1978).

13.4.2 LOUISIANA RESERVOIRS

Water level manipulation is an important method of reservoir management in Louisiana, a southern region of the U.S that experiences some periods of freezing weather in most winters Chemical controls were costly, and harvesting (Chapter 14) was expensive and promoted plant spreading

through fragmentation Water hyacinth biomass (Eichhornia crassipes) can be controlled by drying

and freezing As with milfoil, plants left in a few centimeters of water survive Unfortunately dewatering promotes seed germination, but after 1 or 2 years of drying and freezing there is a significant reduction in viable seeds (Richardson, 1975)

Anacoco Reservoir, Louisiana was drawn down 1.5 m from midsummer to mid-October, reducing reservoir area from 1,052 ha to 526 ha It was refilled by mid-February About 40% of

the reservoir was closed to fishing due to Potamogeton sp and Najas guadalupensis, but after

drawdown and refill, only about 5% of the area was closed The drawdown eliminated water shield

(Brasenia schreberi), restricted the spread of parrot feather (Myriophyllum brasiliense) and water lily (Nymphaea odorata), and enhanced Chara vulgaris (Lantz et al., 1964; Lantz, 1974).

Bussey Reservoir (northeastern Louisiana) was drawn down in October and refilled in May In

the summer prior to drawdown, 280 ha were infested with Potamogeton sp and N guadalupensis.

In the two summers following refill, only 16 ha (40 acres) were infested The treatment was considered 90% effective Lafourche Reservoir, also in northeastern Louisiana, had a partial

draw-down in winter and further water removal in the summer to determine effects on Ceratophyllum demersum (coontail), which infested 80% of the reservoir In the summer following refill, over

60% of the reservoir was clear of coontail (Lantz et al., 1964; Lantz, 1974)

Drawdown in Louisiana is a successful control method for many plants, but cannot be used for eradication However, lake managers could stagger fluctuation years to prevent plants from adapting

to it The recommended schedule is 2 to 3 years of drawdown followed by 2 years without water level fluctuation (Lantz, 1974) Nichols (1975b) also recommended staggered drawdowns for Wisconsin reservoirs Presumably intervals without water fluctuation allow susceptible species to regain dominance over drawdown-resistant species Subsequent drawdown then frees the reservoir once again from susceptible nuisance plants

13.4.3 FLORIDA

A fall-winter drawdown (September 1972 to February 1973) was used to control nuisance vegetation

in a central Florida reservoir, Lake Ocklawaha (Rodman Reservoir) The dominant plants before

drawdown were Ceratophyllum demersum, Egeria densa, Hydrilla verticillata, Eichhornia

cras-sipes (water hyacinth), and Pistia stratiotes (waterlettuce) Water level was lowered 1.5 m and the

study sites were dry or had very shallow water By the end of the second growing season following

reflooding, Ceratophyllum coverage was reduced by 47% and Egeria coverage by 56% Hydrilla

and water hyacinth had a lake-wide increase of 64 times and 33 times, respectively, after reflooding Failure to control these species was due in part to a winter without frost, allowing spread to new areas (Hestand and Carter, 1975) Water hyacinth can be controled by drying and freezing in northern Louisiana reservoirs, although drying appears to enhance seed germination (Lantz, 1974)

In the milder climate of central Florida, conditions appropriate for control of these species through winter drawdown seldom occur

Drawdown to control hydrilla should be based on its life cycle (Massarelli, 1984; Leslie, 1988; Poovey and Kay, 1998) Hydrilla produces dessication-resistant subterranean tubers in early fall Drawdown at this time gives hydrilla an additional competitive advantage and might ensure a

monoculture following refill Release of the weevil Bagous affinis could reduce tuber density

(Buckingham and Bennett, 1994) (Chapter 17) A spring drawdown might be used to kill the standing crop, followed by another drawdown before tuber formation to kill newly sprouted plants (Haller et al., 1976) A third drawdown in the following spring may eliminate remaining plants, a

successful approach in managing hydrilla in Fox Lake, Florida, though cattails (Typha sp.) invaded

dewatered soils making overall success questionable (Massarelli, 1984)

A lock and spillway were built in 1964, reducing the natural water level fluctuations by 71%

in Lake Tohopekaliga, one of the lakes in the Kissimmee chain of lakes in central Florida Shoreline agricultural and housing development, and increased wastewater discharges followed Organic deposits from algae and weeds, particularly water hyacinth, degraded the littoral habitat and fishing success declined A drawdown of 2.1 m from March through September 1971, with final refill by March 1972, exposed 50% of the lake’s bottom The sediments dried and consol-idated, and desirable (for fisheries) submerged plants returned Drawdowns were needed again

in 1979, 1987, and 1990, along with organic sediment removal with frontend loaders and bull-dozers, to maintain the improved lake condition (Wegener and Williams, 1974a; Williams et al., 1979; Williams, 2001)

13.4.4 WISCONSIN

Winter water level drawdown of the 172 ha Murphy Flowage, Wisconsin was successful in opening the flowage (reservoir) to recreation Between mid-October and mid-November, 1967 and 1968, water level was lowered 1.5 m and maintained at that level until March, and then brought to full

volume In 1967, 30 ha were closed to fishing from late spring through summer by Potamogeton robbinsii, P amplifolius, Ceratophyllum demersum, Myriophyllum spp and Nuphar spp The first

winter drawdown opened 26 of the 30 ha for fishing, and none of the above species returned as

dominants in 1969 Megalondonta beckii (water marigold), Najas flexilis, and P diversifolius all increased following drawdown, and P natans was unchanged Even with these resistant species,

there were still 24 of the original 30 ha open to fishing in 1969 (Figures 13.1 and 13.2) Success was attributed to freezing and drying of vegetative reproductive structures in this cold climate region

of the U.S The reduction of the Nuphar population was thought to be due to deep frost and sediment

upheaval The three resistant species were beginning to come back, but the flowage was destroyed

by a flood in 1970, preventing a longer term evaluation Fishing success for largemouth bass increased in summer 1968 (Snow, 1971; Beard, 1973)

The primary negative effect was the appearance of a phytoplankton bloom in August 1968 (Beard, 1973) This response is not uncommon when plants are controlled, perhaps in part due to

P release from reflooded soils and to the absence of the clear water stabilizing effects of macrophytes (Chapter 9) The suggestion that drawdown could be used to switch the lake to the macrophyte-dominated state (Coops and Hosper, 2002) is not supported in this case