The permeability of the Butte Valley Basalt and the volcanic rocks is directly related to the thickness of the individual flows that compose the formations and to the number of individual flows. The thinner flows usually have higher permeability because they cooled more rapidly; thus, fracture and shrinkage joints that result from the cooling are common. Contacts between individual flows are rough, fractured, and porous; more such contacts are in a given thickness of rock that contains numerous flows than one that contains only a few thick flows.
The Butte Valley Basalt is a highly productive, though relatively thin, formation. The thickness of the basalt ranges from 6 feet to slightly more than 100 feet and averages about 40 feet. Individual basalt flows that compose the formation are thin and average only about 12 feet in thickness in one well. The contacts of the flow units are rough and broken and transmit water freely. The thinness of the flow units resulted in vertical fracturing that permits water to move between the flow contacts.
Individual flows in the volcanic rocks of the Cascade Mountains are usually thicker than those in the Butte Valley Basalt, and connections between flows are not as common. However, the volcanic rocks are a dependable source of water, and their large extent provides a vast source. Wells that penetrate several flow contacts can yield large volumes of water.
Local pyroclastic rocks (fig. 133) are as much as 400 feet thick and are moderately to highly permeable. Some of these rocks are above the water table and, therefore, are unsaturated. Where saturated, however, the pyroclastic rocks can yield large volumes of water.
Wells in the eastern and southern parts of the valley obtain large yields from the sand and gravel that compose the upper part of the lake deposits. In the area underlain by the Butte Valley Basalt, wells are commonly screened in the basalt and the underlying sands and gravels of the lake deposits. Lake deposits in the northern, central, and northwestern parts of the valley, however, are not a good source of ground water because of the predominance of clay and silt in those areas.
Alluvium, alluvial-fan deposits, talus, and dune sands in the valley are commonly permeable but usually are unsaturated. Although these deposits are recharge areas for underlying formations, they are not sources of water for wells.