The amount of stream flow, or the pattern and extent of flow fluctuations within the stream reach effects the relative survival or performance of salmonids. Flow reductions or dewatering due to water withdrawals will be included as part of this attribute. This limiting factor can affect all life stages throughout the year.
All streams in the Lapwai basin appear to have been affected by severely altered flow regimes. Hydrologic profiles for this watershed are characterized by low duration, high intensity spring flow events and exceptionally low summer base flow levels. Rheic flow values recorded near the mouth of Lapwai Creek ranged from 1,420 cfs to 1.2 cfs within a six-month period in the first year of habitat monitoring by the Tribe. Discharge data recorded near the mouth of Lapwai Creek from 1975 to 2008 indicates summer base flows have diminished significantly in the last 30 years.
Regional hydrology is thought to have shifted from moderated spring and summer flows derived from prolonged snowmelt periods which peaked in May or June, to the current pattern of intense spring runoff and diminished summer flow produced by rain and snow driven systems which typically peak in March or April. The cause of this shift is likely due to multiple factors, beginning with warmer winters and accelerated snowmelt profiles due to agriculture and forestry practices. High spring flows have been further exacerbated by diminished wetland and riparian vegetation area, increased impervious surface area, an increased drainage network (ditching, roads, culverts), stream channelization and reduced floodplain storage, agricultural activities and timber harvest. These same factors also reduce groundwater recharge, which further diminishes low summer base flow. Summer discharge has also been reduced throughout a number of streams by irrigation withdrawals and domestic water use, while rheic base flow has been further diminished, or lost in many areas due to severe bedload deposition incurred during the intense spring-flow events.
With spring events that provide both periods of extremely high flow (exceeding that preferred for salmonid migration) and greatly diminished flow (below that preferred for salmonid migration), the abrupt hydrology within this watershed can decrease the duration of ‘trigger’ flow for both adult and juvenile migration while potentially dewatering redds located outside of the stream thalweg.
No less important, altered flow regimes are inexorably linked to many of the other limiting factors within this watershed, particularly temperature, habitat complexity, and sedimentation. Summer water temperature, as well as habitat complexity, is affected not only by decreased summer flows, but by channel conditions incurred through extremely high spring flows. Likewise, fine sediment recruitment may increase not only directly through higher spring flows, but through increased shear stresses found under high flow conditions (Rosgen, 1996). Temperature and habitat complexity are also impacted through sustained reductions in base flow incurred through withdrawal of stream flows for irrigation and domestic use, most significantly, those flows diverted from Webb and Sweetwater Creek by the Lewiston Orchards Irrigation District (LOID) for residential irrigation and domestic use.
The Lewiston Orchards Irrigation District (LOID) removes a significant amount of flow from the Lapwai Creek basin via a network of Bureau of Reclamation diversions and canals for residential irrigation and domestic use. The NPT is working closely with LOID and the Bureau of Reclamation to develop Sweetwater and Webb Creek in-stream flow requirements for a NOAA Fisheries Biological Opinion on LOID actions.
Thompson, L. C. (2004). Fish Habitat in Freshwater Streams. San Luis Obispo County Extension, California: University of California ISBN 978-1-60107-291-7. http://anrcatalog.ucdavis.edu/pdf/8112.pdf.