The aquifer is largely composed of permeable coarse sands, gravel, cobbles, and boulders that sit on top of impermeable bedrock (Molenaar, 1988 cited in Caldwell, 2003 ). While most of the aquifer is unconfined, there is a section that is confined in the Hillyard Trough (Kahle 2007). This trough can be found in the aquifer thickness map below on the western side. The specific yield values vary from 0.10 to 0.21 (Hsieh). The hydraulic conductivity for this aquifer varies drastically in different regions of the aquifer, ranging from 4 feet per day to 22,000 (Hsieh). For more on the distribution of hydraulic conductivities, please see the Groundwater Modeling page. "Drost and Seitz (1978) reported transmissivity values that ranged from less than 130,000 ft2/d in the western part of the aquifer to more than 13 million ft2/d near the Washington−Idaho State line" (Kahle, 2007).  In general, due to the highly permeable material and the aquifer being unconfined, water likely travels easily in the unsaturated zone and down to recharge the aquifer. However, there have been several studies as to how the unsaturated zone operates between the Spokane River and the aquifer. "Consistent with earlier studies, they concluded that the river was a losing reach between Post Falls, Idaho, and the Barker Road Bridge west of Spokane (fig. 1) with unsaturated flow conditions between the river and aquifer." (Gearhart and Buchanan, 2000 cited in Kahle, 2005 ).