MICHAEL MANGA: Recent Refereed Publications
Marshall Gannett, Michael Manga, and Kenneth E. Lite Jr,
Groundwater hydrology of the upper Deschutes basin and its influence on streamflow,
AGU monograph, in press
The remarkable stability of flow in the Deschutes River, relative to other rivers with comparable mean discharges, can be attributed to the substantial fraction of flow that originates as groundwater discharge in the upper Deschutes Basin, the region upstream of the Pelton Dam. Indeed, groundwater discharge from the upper Basin provides more than three quarters of the total streamflow for the entire Deschutes Basin. In order to understand the factors that result in such a large component of groundwater discharge and the stability of the flow, we develop a conceptual model for the hydrology of the upper basin. The model is based on the regional geology, the distribution and rate of groundwater recharge and discharge, and measurements of hydraulic head, water temperature and isotopic tracers. We show that three hydrogeologic aspects of the upper basin contribute to the stability of flow in the Deschutes River. First, the large vertical and lateral scale of the groundwater system damp out seasonal and longer period variations of discharge. Second, the high permeability of near-surface rocks, combined with the lack of an integrated surface drainage system, permits high recharge rates and thus large groundwater discharge rates, and reduces surface and shallow subsurface runoff. Third, the high storage capacity of the groundwater system filters out large and abrupt changes in recharge, resulting in greatly subdued changes in groundwater discharge. All three factors are responsible for the absence of serious floods in the upper basin, even during rain on snow events that cause significant flooding elsewhere in Oregon.
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