Hangman (Latah) Creek Watershed Planning Project

Minimum Instream Flow

A watershed and its associated streams depend upon a variety of processes to remain both ecologically and hydrologically functional and productive. These processes are often compromised through land uses such as agriculture, domestic supply, industry, and others.

The Hangman (Latah) Creek watershed is no different. Land uses and other factors have impaired the functionality of the system and reduced its productivity. Competing needs for the water in the system are also causing concern. The greatest need for water and the lowest instream flow levels often coincide in the dry summer months. These conditions prompted the need to assess the availability of water and its uses, and develop recommendations to preserve instream flow levels for all beneficial uses, including fisheries.

The planning unit funded a hydrological investigation to evaluate instream flow conditions primarily for fisheries. Flow recommendations were developed for three levels of habitat protection for resident salmonids. Unfortunately, the planning unit did not have consensus on all elements of a recommendation for a minimum stream flow. The data and progress were submitted to the Washington State Department of Ecology to complete a final ruling. Ecology is scheduled to complete the instream flow ruling process within the next two years. However, even though the planning unit did not agree on all aspects of the project, the study provided good defensible science on instream flow requirements for Hangman Creek.

The geology and climate of the watershed indicate that large increases in base flow are unlikely. However, significant benefits to habitat, particularly for salmon, could be produced with very small increments of flow addition. Each one cfs of additional water in the main stem would add five percent or more to physical habitat values during the low-flow season.

Physical habitat increase alone may not improve salmon potential, because stream temperatures are very warm over most of the distance. Studies indicate that even with the addition of cool water, stream temperatures will improve over only a short distance. Therefore, it appears that flow augmentation would need to be combined with temperature reduction measures to improve fish habitat significantly. These measures include the establishment of riparian vegetation to shade the creek. Improving both flow and temperature conditions simultaneously would provide the greatest benefits. These changes might be possible with restoration in the tributaries and in the upper (Idaho) basin.

It is clear from the analyses that no single action, such as increased flow, will restore salmonid habitat conditions or benefit other water conditions and uses. However, the combined effects of several projects (riparian restoration, upper watershed improvement, increased flows from tributaries) could significantly improve fish habitat and overall health of Hangman Creek.

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