Regional and Local Hydrologic Responses to Climate Fluctuations and Land Use Change, Columbia River Basin, Washington
Duncan, Leslie Lyons
There is evidence that landslides occurred along the White Bluffs in south-central Washington State both within the last 11,000 years and the last several hundred years. Modern, active landslide activity along the bluffs began in the late 1960s. Over the last century, this area of the Columbia River Basin has been converted from rangeland and dryland farming to irrigated agriculture, with water provided to the area via an extensive network of canals and laterals. The most prominent and controversial landslide along the White Bluffs lies above Locke Island. Movement of the slide into the Columbia River has forced the river to shift its flow, eroding the banks of Locke Island and its cultural resources. Water is often implicated as a cause of slope failure. Of particular relevance to understanding the hydrological context for initial failure and longer-term stability of Locke Island landslide are the subsurface recharge and groundwater fluxes at the landslide toe. The influences of land use change and climate fluctuations on the subsurface system, and the relative importance of these factors, has been examined using statistical analysis and a numerical modeling framework. Results from wavelet analysis of an approximately 2000 year Palmer Drought Severity Index dataset indicate that low frequency drought and pluvial cycles, at multidecadal to centennial timescales, are persistent features of regional climate in the Columbia River Basin, which may be linked to solar insolation. Results from a two-dimensional variably saturated finite element model indicate that discharge at the toe of Locke Island landslide is governed by changes to the regional groundwater system. Increases in recharge from irrigation and from a wetter climate can increase regional groundwater flow by raising water levels and increasing the discharge rate from the system. Large fluctuations in river stage can increase discharge at the landslide toe more than regional increases in recharge; these increases, however, tend to be short-lived. Regional recharge to the groundwater system is likely the largest influence on achieving and maintaining a state of equilibrium along the White Bluffs.