There are some unique features of the tailwater fisheries behind dams. Most are good.
Bottom draws provide consistent cold water. The water is rich in dissolved minerals that create a rich biomass of aquatic insects and plant life. Tailwater fisheries act much like giant spring creeks.
However, there are several bad things that can happen. One is nitrogen supersaturation. The water at the bottom of the dam can become super saturated in nitrogen due to the water pressure above it that keeps the nitrogen in solution. When this water is released from the dam, the fish can aborb the supersaturated nitrogen. When they move to an area of lower nitrogen pressure, the nitrogen comes out of solution an bubbles form in their tissues. They get the "bends" and die.
The second risk is winter frazil ice that forms in super cold water and the tiny ice crystals form on the fish gills.
"Warmer water temperatures in tailwaters can prevent formation of stationary ice cover across the channel for long segments downstream from dams (Simpkins et al. 2001a), and contribute to occurrences of anchor ice and frazil ice in these segments (Ward and Stanford 1979). Frazil ice and anchor ice can fill interstitial spaces among gravel and cobble substrates where juvenile fish have sought cover (Stickler et al. 2007a; Stickler et al.
rates, greater swimming ability, and more activity among salmonids in tailwaters, thereby generating a demand on stored energy reserves (Berg and Bemset 1998; Cunjak et al. 1998; Simpkins and Hubert 2000b; Hebdon and Hubert 2001a; Simpkins et al 2003a, 2004a; Finstad et al. 2004b).
Loss of energy reserves can reduce the ability of fish to respond to variation in habitat or threats from predators, thereby enhancing mortality of small fish in tailwaters (metcalfe and Thorpe 1992; Bull et al. 1996; Cunjak 1996; Finstad et al. 2004b). The lack of stationary ice cover associated with warmer winter water temperatures in tailwaters can enhance predation on fish by homoeo- thermic predators such as mink and river otter (Lutra lutra L.; Fraser et al. 1993; Valdimarsson and metcalfe 1998).
Channels downstream from reservoirs often change and lose the complexity that existed prior to construction of the dam due to reductions in extremely-high flows and the lack of sediment released from dams (Ward and Stanford 1979). The result is often a loss of deep pools with low current velocities important to overwintering fish (Stickler et al. 2008b). Reservoirs also affect the occurrence of cobble substrate with interstitial spaces important to juvenile salmonids during win- ter (Rimmer et al. 1984; Heggenes 1996; mäki-Petäys et al. 1997; Linnansaari et al. 2008; Stickler et al. 2008b). Highly-embedded, armored channels downstream from reservoirs generally lack cobbles with interstitial spaces.
Because dams regulate the flows of rivers for a variety of economic reasons, discharge regimes during winter are often quite different from relatively stable natural conditions. Variable discharges to meet hydro- power, flood control, and water storage functions can lead to variable flows during winter, causing substantial variation in habitat at a time when fish need stable habitat (Dare et al. 2001; Lagarrigue et al. 2002; enders et al. 2008).
Variation in flows during winter downstream from reservoirs can strand fish (Saltveit et al. 2001; Berland et al. 2004; Stickler et al. 2007a; Stickler et al. 2007b; enders et al. 2008), force 2007b), remove submerged aquatic macrophytes that are important as sources of cover and protection from predation (Simpkins et al. 2000a; Johnson and Douglass 2009), and force fish to move from normal feeding and resting areas to refuges, such as the bottom of deep pools or under shelf ice in shallow water near shore (Griffith and Smith 1995; Simpkins et al. 2000a; Van Kirk and martin 2000; Stickler et al. 2008a). The movements and lack of feeding opportunities in tailwaters caused by frazil ice episodes accentuate energy demands on fish, affect starvation processes, and perhaps force fish to move downstream out of managed reaches (Brown and mackay 1995; Hebdon 1999), and enhance mor- tality of juvenile salmonids (Simpkins 1997; Simpkins et al. 2000a; annear et al. 2002)."
The article below is a great one on winter changes to trout habitiat: