Major Reservoirs匒re They All Alike?

Did you know there are very real differences between one type of reservoir and another? That certain factors not only control the stability of the fish environment in a reservoir but influence its ability to produce fish?

Getting a grip on a few facts will help anglers know what to expect and perhaps have more success fishing these large bodies of water.

Kentucky has 20 major lakes ranging from 340-160,000 surface acres. These large reservoirs have something smaller lakes don't -- water outlet structures that can be manipulated to permit an orderly release of stored water through gates. (Small lakes usually have an overflow or emergency spillway which allows a continuous discharge of water when it gets above a certain level.)

All of Kentucky's major reservoirs were built prior to 1970, except Taylorsville Lake (1983) and Yatesville Lake (1991). The oldest is Herrington Lake, built by Kentucky Utilities in 1926. These major impoundments were constructed for a variety of reasons including barge navigation, flood control, hydropower, improved water quality and quantity for downstream areas and recreation.

Ebb and Flow
A major reservoir's primary purpose (why it was built in the first place) usually determines how it functions or is operated. Those designed to help control flooding, for example, hold huge volumes of water; they're called storage reservoirs. In late fall, the water level is lowered or drawn down to what's called winter pool. This allows room in the reservoir for storing water from spring rains which helps prevent flooding. Eventually this gradual filling process brings the water level to summer or recreational pool and it's held stable throughout summer. In fall, a gradual decline begins the cycle again.

Lake Cumberland, Laurel River and Herrington lakes are exceptions to the stable-at-summer-pool rule. The reason? The U.S. Army Corps of Engineers and Kentucky Utilities use portions of these lakes' water volume to produce hydropower. Water released throughout summer turns turbines located at the dams and produces the electricity that keeps our air conditioners running.

Mainstream reservoirs, such as Kentucky and Barkley lakes, aren't designed to store large volumes of water. Water entering a mainstream reservoir passes through the dam in a short period of time (around 30 days at Kentucky Lake under normal weather conditions). Most of the water released at Kentucky and Barkley lakes passes through turbines for generating electricity.

The most drastic change in Kentucky reservoirs is fluctuation of the water level (summer to winter pool). On a yearly basis, changes in water levels vary as little as two feet at Taylorsville Lake to as much as 50 feet at Lake Cumberland. These fluctuations are directly related to the volume of water necessary for flood control the primary reason many reservoirs exist. Just how severe drawdowns are depends on the particular reservoir and the shape of its basin. Barren River Lake, for instance, is lowered 27 feet each year. The drawdown reduces the lake's surface acreage 56 percent and decreases lake volume 74 percent.

Established aquatic plants won't be found in large drawdown zones. And over a period of time wave action removes tree stumps in the area. Eventually, woody debris also disappears, a victim of rotting caused by exposure to air.

More importantly, there can he a significant loss of fish through reservoir gates during the actual drawdown period in autumn or during winter when releases are employed to maintain the desired drawdown level or winter pool. Fish species known to readily leave a reservoir during water releases include crappie, white bass and their hybrids, striped bass and walleye.

Shoreline and Water Depth
Most anglers can identify differences in reservoirs' physical features such as water depth and shape of shoreline. A rock-bluff shoreline on Herrington Lake, for example, is obviously different from the extensive mud flats along the shorelines of Barren and Barkley lakes. Also, depending on the shape of impoundments, two reservoirs of similar acreage may have differing amounts of shoreline.

Reservoirs with lots of shoreline and water depths less than 15 feet provide favorable living areas for black bass and other species of sunfish such as bluegill. And shallow-water habitat is further enhanced if there is fish cover such as aquatic plants, tree stumps, rock outcroppings or artificial structures.

Fertility
Differences in water quality and fertility are less obvious to the casual observer but vital to producing fish. The fish food chain begins with nutrients (nitrogen and phosphorus) carried to the reservoir from tributary streams. Nutrients and sunlight are necessary to produce plankton (tiny animal and plant life), the bottom of the food chain and required food for all fish at some life stage. Generally, the amount of plankton in the water translates into the numbers of fish that can be produced in the reservoir. Interestingly, shallower reservoirs typically produce more fish per acre than deeper impoundments, provided nutrient levels are the same.

Clear water, in which objects may be seen 10 or more feet below the surface contains fewer plankton and, therefore, has a lower fertility level than green colored water. Green water usually indicates a plankton bloom and a higher fertility level due to more nitrogen and phosphorus in the water. Muddy water, caused by rain runoff from mining, construction or agricultural activities in the watershed, can interfere with plankton production because sunlight can't penetrate the water column.

Reservoirs are classified according to three fertility levels (1) fertile or eutrophic; (2) moderate fertility or mesotrophic; and (3) infertile oligotrophic.

Fertility classifications are based on the overall reservoir, however, individual embayments or sections of the lake may have a different status. In general, reservoirs located in high agricultural use areas will have higher nutrient levels and a higher fertility classification than impoundments surrounded by forested land.

Water Layers
In summer reservoir water begins to stratify, forming layers varying in depth and thickness. The layers form based on temperature, with the coldest water near the bottom and the warmest nearest the surface. (Many anglers are familiar with the term "thermocline," the transition zone of declining water temperatures between warm and cold water.) The depth of these layers depends on several factors: depth of the reservoir; depth and type of the water release; and the water exchange rate (how fast water is replaced in the reservoir by the inflowing river).

The amount of dissolved oxygen (a necessary ingredient for fish) in the layers is influenced by lake fertility. Deep infertile reservoirs contain both cool- and cold-water zones with suitable oxygen levels for fish to survive. This is generally not the case in shallow, fertile reservoirs. Cold-water zones with sufficient oxygen are necessary habitat for cold-water species such as trout to survive through a Kentucky summer. Equally important are oxygen rich, cool-water zones which favor species such as muskie, walleye, striped bass and smallmouth bass.

Contrasting examples are a couple of deep reservoirs, Herrington Lake (average depth 78 feet) and Laurel River Lake (average depth 72 feet). Both have deep cold-water ones during summer, however, Herrington lacks dissolved oxygen below 15 or 16 feet, hut in Laurel oxygen extends 40 to 50 feet into the cold-water zone. Herrington is a fertile lake while Laurel is classified infertile.

Weather and Fish
It seems residents accept Kentucky's variable weather but may overlook the impacts these on-again-off-again conditions have on fish production in reservoirs. Two examples come to mind which demonstrate how varying climatic conditions come into play. The severe winter of 1977-78 held an icy grip on Kentucky and all its reservoirs except lower-sections of Herrington and Cumberland lakes. The ultimate result of ice-covered lakes was extensive shad loss which limited the fish food supply for several years. Then the drought years came along in the mid-1908s which meant less water runoff, resulting in fewer nutrients going into many impoundments. Lack of spring rains also meant there were fewer numbers of some fish species which migrate upstream to spawn such as sauger and white bass.

The average fish-growing season (defined as frost-free days) equals 180 days in Kentucky's central and western reservoirs and 160 days for eastern lakes. This represents a 10 percent difference between regions. Obviously, the fish-growing season is longer in reservoirs south of Kentucky.

The Bottom Line
As you can see, many factors influence a reservoir's capability to produce fish. Some studies show nutrient levels and average water depth are the most important variables influencing fish production. Translation: shallow, fertile reservoirs produce the most fish. However, high water exchange rates (which depends on rainfall for the year) have a profound influence on yearly fish production and the number of fish lost through the dam. And these variations in fish populations translate into how many fish will be available for anglers to catch.

The bottom line is the Kentucky Department of Fish and Wildlife Resources must manage fisheries within the confines of each reservoir's unique characteristics and capabilities- AND ACCEPT THE FACTORS THAT CAN'T BE CONTROLLED -while, simultaneously, maximizing opportunities to enhance sport fishing through stocking, regulations, enforcement and any habitat or water level manipulations practical to implement.