What are instream flow assessments for?

Instream flow assessments aim to determine the minimum flow in rivers and streams that will provide the desired level of protection for the aquatic environment. This protection can be described in terms of the proportion of historic flows, the wetted area, or the area of suitable habitat for a particular species. The historic flow method, also known as the Tennant Method, assumes that a certain percentage of the mean annual flow is required to maintain a healthy stream environment. This method is based on the observation that stream width, water velocity, and depth all increase rapidly from zero flow to 10% of the mean flow, and that the rate of increase declines at flows higher than 10%. The hydraulic method predicts water depth and wetted channel width throughout a reach as discharge is varied, and it is used to optimize the quantity of habitat created. The habitat method, on the other hand, focuses on the quality of the habitat and determines the flow required to match the specific biological requirements of fish and other organisms. If there are conflicting minimum flow assessments from these different methods, it may be because they are based on different environmental goals and levels of protection.