Tailwater refers to waters located immediately downstream from a hydraulic structure, such as adam,spillway,bridge orculvert.[1] Generally measured and reported as the average water depth downstream of a hydraulic structure, tailwater can vary based on the outlet from the structure as well as downstream influences that may restrict or advance the usual flow of water from the structure. The creation of a tailwater will have significant impacts on both theabiotic andbiotic conditions of the waterway.
The environmental conditions in a tailwater influence the entirefood web of the waterway. Consistent flows, higher temperatures, and clear water found in tailwaters create an ideal habitat for filamentousgreen algae.[2] The near-shore zones of tailwaters that are submerged during hydropeaking but dry during consistent flows are far less productive areas of the waterway. Most algal species are not adapted to handle this exposure to air for extended periods of time resulting in a loss ofprimary productivity and an increase in organic matter that gets washed away when flow increases.[2]
Macroinvertebrate communities assemblages largely depend on algal composition of the waterway. With the conditions created by hydropeaking in tailwaters, much of the macroinvertebrate diversity is lost resulting in a few dominant taxa.[3] For example, theColorado River experiences dramatic flow changes due to the abundance of hydroelectric dams on the river. Below the major dams, between three and seven macroinvertebratetaxa account for over 95% of the total abundance in the system. Nearly half of these species are specialists who spend their entire lives underwater and do not provide energy to the surrounding terrestrial environment.[4]
Native fish species are particularly endangered by tailwaters. The construction of the tailwater itself is a severeenvironmental disturbance that can reduce richness, but immediately following dam completion, many native fish species can still be found. The long-term impacts on native fish communities, especially in historically warmwater systems, are especially dramatic. Before the completion of the Beaver Dam on the Ozark River, 62 species of fish from 19 unique families were present in the system. Following dam construction, only 18 species from 8 families were collected, with two families accounting for over 90% of the fish found. 30 years after the installment of the impoundment, 28 species of fish were sampled from 8 families, however 98% of the fish were eithersculpin ortrout species, both of which were essentially absent prior to the creation of the tailwater.[5] This lack ofspecies evenness results in an altered aquatic ecosystem that is far from the community makeup seen before the impoundment was in place. Substrate composition in tailwaters can also impact native fish survival. Some species, such asriver chubs, construct nests from gravel and rocks found on the river bottom. Tailwater flows can alter substrate size making it difficult for native fish to buildspawning areas. Increased flows can also flatten nesting sites or scour eggs from the nest making spawning in a tailwater system more difficult.[6]
The construction of a dam will often change the makeup of the body of water immediately upstream and downstream of the structure. In many cases, the water that comes out of the dam originates from the bottom of thereservoir created by the impoundment. The resulting damdischarge is relatively cold compared to the natural temperature of waterway due to the stratification of water that occurs in reservoirs.[7] The resultingthermal pollution can have devastating impacts on native fish assemblages.
Tailwaters are also subject to changes in traditional flow rate. Some impoundments discharge a consistent amount of water which can disrupt seasonal fluctuations and extreme flow events. On the other hand, hydropeaking, the cyclical increase in discharge below a hydroelectric dam to meet power demands, can rapidly increase tailwater flow rates. The dramatic changes in river flow can scour the river bottom, change river velocity and depth, and reduce biotic richness in the waterway.[8]
Tailwater can also refer to a type offishery. Fishing in tailwaters can be very productive due to consistent water temperature and flow rates found below an impoundment. Nutrients from the lake upstream are released into the tailwater, creating a productive environment in which target fish species, usually trout, can thrive.[9] An example of this phenomenon is the fishery atLee's Ferry on theColorado River inArizona.
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