Tailwater is the body of water located immediately downstream from a major hydraulic control structure, such as a large dam or a hydroelectric power facility. This term specifically refers to the river section that receives the regulated outflow from the reservoir, beginning at the point of release. The characteristics of this water are directly influenced by the structure and its operation, creating a unique aquatic environment. Understanding the features of this regulated environment requires a focused look at how the water is sourced and released.
The Hydrological Origin of Tailwater
The existence of tailwater is a direct result of a dam impounding a river and creating a large reservoir. The mechanism by which the water is drawn from the reservoir and channeled back into the river dictates the initial conditions of the tailwater environment. Dams are generally designed with either a surface-release or a bottom-release structure to manage the flow, and this choice is responsible for the most significant alterations to the downstream ecosystem.
Deep-water release dams are the primary source of the most impactful tailwater environments. These structures draw water from the bottom layer of the reservoir, known as the hypolimnion, often through an intake pipe called a penstock, which then feeds into the tailrace below the dam. By contrast, a surface-release dam draws warmer, oxygen-rich water from the reservoir’s surface layer, or epilimnion, resulting in a less dramatic change to the downstream river’s natural temperature profile.
The reservoir acts as a large settling basin where thermal stratification occurs, creating distinct layers of water based on temperature and density. Water released from the deep, cold hypolimnion creates the signature characteristics of many tailwaters. This regulated outflow is entirely disconnected from the natural seasonal or weather-driven fluctuations of the original river.
Distinct Physical and Chemical Properties
The source of the tailwater from the deep reservoir fundamentally alters its physical and chemical composition compared to a natural river. The most immediate physical change is the water temperature, which is consistently cold year-round. This is because the hypolimnion water is shielded from solar warming and maintains a temperature near 4 degrees Celsius, regardless of the season.
This consistent thermal profile is a form of thermal pollution, replacing the natural seasonal temperature cycle with a monolithic, unnaturally cold regime. Another physical property is the highly regulated flow, often characterized by hydropeaking—the cyclical release of water for hydroelectric power generation. This schedule causes rapid, artificial surges and drops in flow disconnected from natural precipitation events.
Chemically, the tailwater is clearer than the natural river because the reservoir acts as a sediment trap, causing suspended particles to settle out. This results in lower turbidity downstream, which changes the amount of light reaching the riverbed. However, water released from the deep reservoir layers frequently has a lower concentration of dissolved oxygen (DO) due to the decomposition of organic matter in the hypolimnion.
Low dissolved oxygen can lead to hypoxic conditions, especially in the immediate tailwater section. Anaerobic conditions in the deeper reservoir can also cause the dissolution of certain metals, such as iron and manganese, which may be present in higher concentrations. These altered physical and chemical measurements define the new, regulated aquatic habitat.
Ecological Impact on Downstream Biota
The distinct properties of tailwater create an altered ecosystem with unique biological consequences. The combination of consistently cold water and clear conditions often favors non-native, cold-water fish species, transforming the river into a highly productive cold-water fishery, such as those known for trout and salmon. This habitat alteration results in a species-limited environment, as the constant cold temperatures exclude native warm-water fish species that rely on a natural thermal cycle for spawning and growth.
The unnatural flow regulation from hydropeaking introduces significant stressors for native biota. The sudden, large fluctuations in water level can physically scour the riverbed, damaging or dislodging aquatic invertebrates and their habitats. These flow pulses disrupt the life cycles of organisms that evolved to rely on natural seasonal cues for reproduction and migration.
The base of the food web, which consists of aquatic insect life, is particularly vulnerable to the altered conditions. Macroinvertebrate communities in tailwaters often exhibit reduced diversity, with only a few dominant taxa that are resilient to the daily flow variations and the unnatural water temperatures. This shift in the invertebrate community affects the availability of food for fish and other predators.
Additionally, the unnatural flow regime affects riparian vegetation along the banks, as the constant cycle of submergence and exposure limits the productivity of near-shore zones. While the clear water can promote the growth of filamentous green algae on the riverbed, the overall ecosystem has been fundamentally reorganized, favoring a few specialized species that can tolerate the regulated flow and temperature over the diverse native community.