|Abstract or Summary
- Columbia River Basalt Group (CRBG) aquifers are commonly thought to be relatively flat lying, laterally extensive, and mostly confined, and therefore not likely to be directly connected to surface water. However, many of the CRBG units in south-central Washington and north-central Oregon were deposited within evolving synclinal structures of the Yakima Fold Belt. The synclines hosted drainage systems that interacted with the encroaching lava flows, resulting in enhanced porosity and permeability where the flows encountered water or wet sediment. The geometry of the synclines also constrained the lateral extents of the lava flows.
Modern streams within the synclines have exposed some of the permeable flow contacts and locally allowed connectivity between the streams and aquifers. Where the elevation of the hydraulic head in the aquifer is above the elevation of the exposed contact, the aquifer typically discharges to the stream. Historically, this was the case in Mosier Creek.
In confined (S= 10-4 – 10-5) and syncline-controlled ground-water flow systems like that near Mosier, seasonal and long-term changes in hydraulic head in aquifers can propagate rapidly to stream/aquifer boundaries. For example, as hydraulic head in the Mosier groundwater flow system declined over 150 feet in the last 4 decades, Mosier Creek has gone from a gaining stream to a losing stream in the vicinity of the exposed contacts. The exposed contacts in Mosier Creek represent head-dependent boundaries to the Mosier groundwater flow system.
Hydraulic head decline in the Mosier area is due to depressurization of aquifers by interaquifer flow through uncased sections of wells, and overdraft conditions in one of the aquifers. But regardless of the decline mechanism, any head decline in a CRBG aquifer connected to a gaining stream will result in diminished streamflow.