What is the Natural Flow Regime?

A "natural flow regime" refers to a river's naturally occurring changes in water flow through the course of the year.

The changing quantity of water flowing through a river - its "hydrology" - significantly influences water quality, temperature, nutrient cycling, oxygen availability, and the geomorphic processes that shape river channels and floodplains. A river's natural flow regimen varies throughout the course of a year but also from year to year.

The fluctuations in flow are important to the ecological health of a river system. For example, the spring floods deposit nutrients on the river banks and strip bare new cobble beds which become the breeding ground for new cottonwood groves.

The Bow River is a snowmelt river with a natural seasonal pattern. The peak flow period occurs in the spring. These changes in flow are caused by the progressive melting of the snowpacks at higher and higher elevations as the mountains heat up. After the snow melts in the mountains and the peak flows subside, river flows decline in the late summer to much lower flows in the fall and winter. After the spring runoff, river flows decrease, and glacial melt and groundwater make up a larger portion of the river's flow. The natural flow regime cycle begins again with the snow melt the following spring.

Dams tend to make fluctuations in river flow less dramatic, capturing water at high flow times, and releasing it in times of less flow. But it is possible to operate dams so they mimic some of the natural flows that a river needs.