Center for Coastal Resources Management

Resources: York River Water Budget - Comparing Methods in Rivers and Estuaries

Biological method:

1. most flexible of the three methods.

2. considers the needs of multiple species or life history stages and the most commonly used method in the U.S. 

3. compares water depth and velocity with the habitat suitability requirements for a species of interest over a variety of flows to determine how suitable habitat area varies with flow and the capability of the flow regime to support spawning, feeding and passage of fish. 

4. preserves habitat in terms of depth and velocity, but does not necessarily preserve river character.  The relationship between habitat area and flow is generally non-linear, so flow recommendations are set at the threshold above which diminishing returns are realized. 

5. in situations where the relationship between habitat area and flow is linear, the resulting flow recommendations are similar to the hydrologic and hydraulic flow methods.

Disadvantages: its focus on a target species may fail to consider other, important aspects of the river ecology and that the resulting minimum flow recommendations are dependent on stream size, with higher minimum flows requirements in smaller streams.

There are many key differences between rivers and estuaries which argue that different methods may be necessary for determining minimum instream flow in each system.  Some of the differences are summarized in the table above (modified from Pierson et al. 2002)

In estuaries:

1. it may be better to target habitats (eg. marshes, SAV) which are known to be beneficial to a wide diversity of species and life stages 

2. flow requirements in estuaries must also take into account the changing conditions prevalent in estuaries, such as current and predicated rates of sea-level rise and increased eutrophication and sediment inputs associated with watershed development