Tailwater refers to waters located immediately downstream from a hydraulic structure, such as a dam (excluding minimum release such as for fish water), bridge or culvert. 
Tailwater can refer to a type of fishery. Tailwater fisheries can be defined as Tailrace fishing which occurs at the outflow from large dams, where the size of the reservoir creates a steep temperature gradient, with colder water stored at the bottom of the reservoir near the outlet. The constant cold-water flow provided by the reservoir's outflow, coupled with the generally silt-free nature of the outflow, creates ideal water conditions for cold-water fish such as trout in environments that ordinarily might not support a robust trout population. An example of this phenomenon is the fishery at Lee's Ferry on the Colorado River in Arizona.