Operational changes refer to how projects can regulate either 1) the rate of water flow in a river, or b) how water passes through a project.
Run-of-river projects and those with fairly small reservoirs have limited or no ability to regulate the rate of water flow in a river. The reason is that the water is already flowing at or near the river’s natural flow rate.
At projects with reservoirs, the most common operational changes involve “flow augmentation” or “permanent drawdowns.” These changes commonly result in lost capacity to generate power when it is in highest demand and most valuable.
Strategy: Flow Augmentation
Issues: Fish Migration, Fish Populations, White Water Boating
One of the factors in juvenile salmon mortality is the length of time it takes to migrate to the ocean. To reduce the in-river time of this journey, a “water budget” has been used to release water from the largest upstream storage reservoirs in the Snake (Dworshak) and Columbia (Lake Roosevelt). When this occurs, water not passing through a turbine area is released over a dam’s spillway.
Water is stored and then released in concert with juveniles migrating downstream in the Spring. By increasing water flows and “flushing” juveniles downstream, another advantage may be that more natural freshet conditions during migration are duplicated. Freshet refers to the turbidity and other water conditions that occur when water is flowing rapidly downstream as a result of conditions such as rainfall, snowmelt, or sudden release of water from a reservoir.
From a recreational perspective, the timing and length of spills can dramatically affect conditions for white water rafting, kayaking, and similar activities.
Strategy: Permanent Drawdowns
Issues: Water Temperature, Stratification, Oxygen Levels, Fish Migration, Fish Populations and White Water Boating
Permanent drawdowns speed up the rate (velocity) of water flowing down the river by lowering the water level at a project’s reservoir. In effect, it largely or completely restores a river to its “natural” state. By doing so, a project loses all or most of its ability to store water and thus regulate the generation of electricity to times when it is most needed and valuable. Rather than being or remaining a storage project, it becomes a run-of-river project.
How much to draw down reservoirs has been the subject of much debate. One issue is that while new rearing and spawning habitats for some species will emerge, habitat for other fish and wildlife will be lost. Other issues include erosion and possibly needing to reconstruct fish passage and recreation facilities due to reduced water levels.