Determining Environmental Flows for Flushing of Fines from Rivers: Cache la Poudre River, Colorado USA

The quantification of streamflows needed to maintain an acceptable level of river ecosystem quality is an important issue in the management of water resources. Part of the environmental (ecosystem) streamflow required to maintain a healthy channel for desirable aquatic organisms is a flow that maintains the stream bed. This environmental flow need is of two types: one to flush sand and fines from the substrate; and the other to maintain a desirable channel form. An approach to the calculation of flushing flows needed to remove sand and fines from the bed of a gravel/cobble bed river is the topic of this paper. The task is to review methods of determining flushing flow requirements and then to present an approach to determining the streamflow needed to remove sand and fines from the bed-surface in regions of the stream that are important for the aquatic ecosystem. The various methods are described by calculating the flushing flows required for the Cache la Poudre (Poudre) River in north central Colorado USA. The Poudre River is a gravel/cobble bed river. After describing existing methods, an approach is presented that could be used to determine the flushing needs for areas of importance to an aquatic organism. The flushing flow criteria are functions based the median sizes of both the armour and the sediment to be removed. These functions are used to determine the dimensionless shear stress (τ*) required to remove the sand and fines. The calculate τ* uses the equation τ* = 0.0346 (Dp/D50)^0..30 where Dp is the size of the sediment to flushed from the bed-surface and D50 is the median size of the bed-surface material. Also presented is how to link physical habitat to aquatic animals to the flushing flows. The presentation on the links between physical habitat and the need for flushing of fines and sand shows that not having flushing flows can be a significant limit on the physical habitat for aquatic animals.