Hangman (Latah) Creek Watershed Planning Project

Multipurpose Storage

The Hangman watershed has low to moderate precipitation (19-40 inches) of which a significant portion is lost to evapotranspiration (e.g., >75%). Much of the precipitation falls during the winter as snow. Stream flow conditions develop from rain falling on snow (and frozen ground) that causes flashy flows with flooding during the spring and low flows during the summer. Groundwater recharge and groundwater supported stream baseflows are low.

Overall, there is little natural water storage capacity in the watershed. Land use patterns have modified the majority of the basin from natural camus prairie vegetation to dryland crop agriculture. The effect of these land use patterns has been to further reduce the intrinsic water storage capacity of the watershed and accentuate the flashiness of the hydrologic regime, causing higher peak flows and lower summer flows, along with accelerated sediment erosion.

No one storage option will completely satisfy the wide range of physiographic features and needs of the Hangman watershed, so multiple options may be the appropriate method to enhance the quantity of water for consumptive and in-stream needs. Studies done in the watershed indicate that significant gains could be produced with small increments of flow addition. Each cfs of additional water would add 5 percent or more to physical habitat values during low flow conditions. This relates to the primary goal of this multi-purpose storage assessment – to increase summer low flow conditions.

To provide direct comparisons among water storage options in WRIA 56, the options were reviewed under the context of their ability to attain a standard value of 600 acre feet of water storage. A storage volume of 600 acre-feet can sustain a streamflow augmentation of approximately 3 cfs for three months.

The most cost-effective options for increasing flow are to drill or pump new and existing wells to augment the streamflow with groundwater and wetland restoration. However, these options will only augment flows in the lower and middle portions of the watershed.

Only three major storage options provide streamflow augmentation to all areas of the watershed. These three options include the use of catchment basins to capture and store water in the upper watershed, developing balancing basins in the upper watershed to capture and store runoff during peak periods, and creating Smith Creek Dam. These are however, significantly more costly options to implement.

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