A MODEL FOR DISCHARGE IN SPRING-DOMINATED STREAMS AND IMPLICATIONS FOR THE TRANSMISSIVITY AND RECHARGE OF QUATERNARY VOLCANICS IN THE OREGON CASCADES

A model for discharge in spring-fed streams is described and applied t o streams in the Oregon Cascades. These streams are assumed to be fed by an unconfined aquifer composed predominantly of quaternary basalts and basaltic andesites. The model is based on the unsteady Boussinesq equation and is characterized by a single parameter, a normalized leng th scale of the aquifer. Flow in a runoff-dominated stream is used as a proxy for the time-dependent groundwater recharge. Four spring-domin ated streams with 50 years of daily discharge records are studied, and modeling efficiencies are in the range of 0.76-0.89. The effective tr ansmissivity is found to be approximately proportional to the length s cale of the aquifers. Groundwater recharge rates are in the range of 6 6-127 cm/yr and 40-73% of the mean annual precipitation.