The structural and hydrogeology of Dishman Hills Conservation Area was interpreted to gain a better picture of its interaction with the Spokane Valley-Rathdrum Prairie aquifer. The rock in this area is Precambrian Ravalli Group with granitic migmatites. This is due to exhumation causing decompression melting in between the bed layers. Other rocks such as amphibolite, gneiss, schist, quartzite, and phyllite are seen along with granitic dikes (WSDNR). This rock type covers all of the Dishman area and is Precambrian in age. Due to tectonic forces in the Spokane area, fractures have also formed within the Precambrian rock. Water will concentrate in fracture zones, known to be areas of groundwater drainage. Many studies show that the water yield on a fracture trace is much greater compared to yields not on fracture traces, with the greatest yield found at intersecting fractures (Fetter, 1994). Using lidar and field measurements, fracture orientations were measured and compared to the stress regime from Hammond (2013). Precipitation and evapotranspiration data were then used to estimate the amount of water that would enter the Spokane aquifer in non-ponded areas. Three-point problems were also created to see groundwater flow direction and to note if water exits these fractures. The calculations suggest that water tends to flow away from the Dishman Hills area. Also, it was found that the groundwater leaving the Dishman Hills area towards the Spokane Valley-Rathdrum Prairie Aquifer ranged between 2.37∗10^7 and 5.92∗10^7 gallons.
McGregor, Cole R., "Analysis of Dishman Hills Structure and Hydrogeology" (2021). Geosciences Student Work. 1.
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