Management Plan for the Center Hill Tailwater Trout Fishery 2004-2009
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Management Plan for the Center Hill Tailwater Trout Fishery 2004-2009 Prepared by: Frank C. Fiss David W. Young Tennessee Wildlife Resources Agency December 2003 Management Plan for the Center Hill Tailwater Trout Fishery 2004-2009 Prepared by: Frank C. Fiss and David W. Young Tennessee Wildlife Resources Agency December 2003 Approved by: Introduction In recent years the Tennessee Wildlife Resources Agency (TWRA) has committed substantial resources to manage and evaluate the trout fishery below Center Hill Dam. This fishery serves a unique role in middle Tennessee, being one of few trout fisheries in the area that can accommodate thousands of anglers each year. The purpose of this plan is to identify TWRA’s goal, objectives and management strategies for the management of this fishery. Goal TWRA seeks to enhance the quality of the trout fishery for the variety of anglers that fish the Caney Fork River downstream of Center Hill Dam (Center Hill Tailwater). Objectives 1. Provide fisheries for rainbow trout and brown trout capable of sustaining at least 25,000 trips during March through September annually, 2004-2009. This would allow for about a 20% increase in fishing pressure compared to recent surveys, which averaged approximately 21,000 trips. 2. Maintain angler catch rates greater than 1 trout/hour as measured by creel surveys from the dam to Stonewall. Despite potentially higher use, we would like to maintain this catch rate, which is typical of Tennessee tailwaters. 3. Increase the abundance of quality (>14 inches) of brown trout by January 2009. Specifically, this objective will be met when the winter electrofishing capture rate for 14+-inch trout exceeds 2.0 fish per site. Current Status Discharge (Flow) Center Hill Dam was constructed by the U.S. Army Corps of Engineers (USACE) in 1948 for the purpose of flood control and hydroelectric power generation and is located in DeKalb County, Tennessee, about 70 miles east of Nashville. The impounded Caney Fork River, and its tributaries, makes up the 23,606-acre Center Hill Reservoir. The Nashville District of USACE operates the dam. The tailwater section of the Caney Fork flows in a northwest direction for about 26 miles before joining the Cumberland River near Carthage, Tennessee. The Smith Fork is a major tributary that enters about 9 miles below the dam. At base flow the Smith Fork has 30- 100 cfs, commonly increasing to 1000-3000 cfs during storm events. The daily generation schedule typically follows the peak demand for power, which occurs in morning and evening in the winter, and afternoons in the summer. The dam has 3 turbines each providing 3,500 cfs during average loading. With three turbines operating, the water level immediately below the dam can rise about 10 feet within an hour. This effect gradually decreases with distance from the dam; at Stonewall, 16 miles downstream, the river rises only about 5 feet when the pulse arrives. During no generation periods a minimum flow of 60-90 cfs occurs through seepage around the dam. 1 The leak in the dam does not provide enough minimum flow to cover all channel habitat. Dewatered habitat is essentially out of production. Additional minimum flow would greatly benefit the upper 5 miles of the tailwater where water level fluctuations are the greatest. Invertebrate populations cannot colonize shoal habitats because they dry out almost daily. Those that do become established during wet periods subsequently die during dry spells. Increased minimum flows would provide additional stable habitat for macroinvertebrates and trout, and thereby increase the numbers of both in the river. Minimum flows have improved the macroinvertebrate communities in tailwaters below Tennessee Valley Authority (TVA) projects (Scott et al. 1996). In the late 1990s, a weir dam was proposed by USACE that would impound 1.5 river miles immediately downstream of the dam and provide an estimated 200 cfs continuous flow. An alternative to the weir was a turbine bypass system, or Valve Engineering System (VES), which would also allow 200 cfs of continuous flow. TWRA favored the VES system, opposing the weir due to concerns of habitat loss. While 200 cfs has been proposed as a target minimum flow, additional work needs to be done to determine the best flow needs for aquatic life and angler access. Water Temperature Intermittent generation provides coldwater but also results in substantial temperature fluctuations. During generation, hypolimnetic discharge delivers cold water (10-15 oC). Despite solar warming and warmer water from the Smith Fork, the first 16 miles below the dam is cool enough for trout (< 18 oC). The remaining 5 to 10 miles can typically support trout provided that generation frequency or magnitude is sufficient. During warmer seasons the water temperature can fluctuate drastically with generation, at times fluctuating from 10oC to 20oC and back to 10 oC (Ramachandran and Gordon 1986). Rainbow trout can acclimate to temperature and survive declines of 12 oC per hour (Cherry et al. 1975). However two or more days are needed to acclimate to warmer temperatures and acclimation to cold temperatures may take three or more days (Peterson and Anderson 1969; Javaid 1972; Evans 1990). Fluctuations in temperature can adversely affect growth rates of trout (Becker et al. 1977; Hokanson et al. 1977). As temperature approaches these lethal levels, trout can experience reduced growth rates and stress. And over time both can affect the survival of trout. The upper lethal temperature limits for brown trout and rainbow trout are 21 oC and 25 oC, respectively (Walburg et al. 1981; Black 1953). Long periods without discharge could increase temperature above the lethal limits, especially in the lower river. In 2002 USACE modified typical operating procedures to alleviate the potential for warming the lower river. On consecutive summer days when less than 2 hours of generation are scheduled the potential for warming is greatest. To address this issue an additional hour of generation was scheduled to interrupt the long no-generation period. The additional hour of generation was scheduled in the morning (around 7 am) and produced the expected benefit. However the additional generation disrupted fishing plans of many anglers that would otherwise get to fish in the morning. USACE is developing a model to determine on which days the 2 additional pulse is necessary, and at what time of day it would be best to generate the additional pulse. Dissolved Oxygen (DO) The Center Hill turbine intakes are located 30 meters below the surface at normal summer pool. Deep reservoirs like Center Hill will thermally stratify during the summer yielding cold water at this depth most of the year. However due to the nutrient loading from five major municipalities combined with the inherent limnology of deep reservoirs, DO concentrations at the turbine intake depth is typically low in the fall. As a result, generation during September through November releases water with DO concentrations that can be less than 2.0 parts per million (ppm) immediately downstream of the dam (Figure 1). Depending on initial DO levels, it can take up to 16 miles to fully recover DO to 6 ppm (Figure 2). Fall anoxic conditions in the tailwater are usually worse in years following heavy late winter or spring rains. High rainfall requires high discharge that depletes the storage of oxygenated water at the intake level. Center Hill Lake Dissolved Oxygen Releases 2002 12.0 11.0 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 Dissolved Oxygen, ppm 2.0 1.0 0.0 Dec-25 Feb-13 Apr-4 May-24 Jul-13 Sep-1 Oct-21 Dec-10 Jan-29 Figure 1. Dissolved oxygen (DO) concentration immediately below Center Hill dam during 3-turbine generation. Data collected in 2002 by USACE. 3 Center Hill Tailwater Reaeration Study Observed DO Values 7.0 6.0 TN Minimum Standard = 6ppm 5.0 4.0 3.0 Dissolved Oxygen, mg/L 2.0 1 units @ 45MW 3 unit @ 120MW 1.0 0.0 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 Distance from Center Hill Dam, miles Figure 2. Recovery of dissolved oxygen below Center Hill Dam during 1-turbine and 3-turbine generation, on 10/4/2003 and 10/7/2003, respectively. Data collected by USACE. Low DO concentrations have adverse impacts on trout survival, health, growth, and catchability. Trout die at DO levels less than 2.5 ppm (Davis 1975), especially when exposure lasts more than 24 hours (Doudoroff and Shumway 1970). In addition to direct mortality, sublethal effects have also been noted on trout populations. Reported minimum threshold levels for rainbow trout range from 4.6 to 8.7 ppm with a median at 6 ppm (Davis 1975). In the Norfolk Tailwater, Arkansas, Todd and Bly (2000) reported poor health and condition of rainbow trout in the upper reaches of the tailwater where DO levels were periodically below 6 ppm. On Lake Taneycomo, Missouri, for each decrease of 1 ppm between 6.0 and 2.4 ppm resulted in a 0.1 trout/hour decrease in angler catch rates (Weithman and Haas 1984). The State water quality standards require 6 ppm of DO for trout waters, even for artificial tailwaters (TDEC 2000). USACE has taken steps to improve DO below Center Hill Dam by releasing water through the sluiceway, altering the loading on turbines, adding hub baffles, and turbine venting. Diverting some oxygenated water down the sluiceway increased DO but also increased water temperatures to a temperature that was lethal to trout. Operating turbines at ½ and ¾ load did not improve DO levels enough during critical times. By December 2001, all units were equipped with hub baffles and turbine vents.