Uncertainty in Minimum Instream Flow Requirements for Streams in Semi-Arid Environments

Increased competition for water resources in arid and semi-arid watersheds is prompting management agencies to adopt instream flow requirements for critical aquatic habitats. For instance, because of increasing concerns for bull trout and salmonid species in the Touchet River system, minimum instream flows are needed to protect several important rearing and spawning reaches. The study included a field reconnaissance of the region with specific emphasis on known migration blockages, spawning and rearing habitat areas, and other areas identified in the project scoping meeting. Eight representative cross sections were selected at each of seven stream segments based on this initial field reconnaissance survey. Care was taken to include pool, spawning, riffle and other unique stream characteristics. Depth and velocity profiles were measured at each of the cross sections at three different water stages: high, medium and low. Substrate grab samples were taken along each of the eight reaches for subsequent analysis. Temperature data along with several other basic water quality parameters (i.e. dissolved oxygen, pH, conductivity and turbidity) and an assessment of vegetative cover were also recorded during sampling. Minimum instream flow modelling and analysis were conducted using the instream flow incremental methodology technique. Habitat suitability indices were assigned to each stream cell. Non-binding recommendations for instream flow values were provided to management agencies. A considerable amount of uncertainty exists in the preference factors assigned to the various life stages of bull trout and salmonid species. Much of this stems from the processes traditionally used to determine factors related to juvenile rearing, spawning and migration. A conclusion of this study was that preference curves developed for wet regions were not applicable to semi-arid river basins where stream flows are often quite small in comparison. This study documents the assumptions, procedures and results of this investigation and demonstrates the potential impacts of uncertainty on the results.