EVALUATING EXTREME FLOOD POTENTIAL IN ADJACENT BASINS, UPPER BOISE RIVER BASIN, IDAHO

Paleohydrologic investigations of extreme floods in adjacent basins of the upper Boise River Basin, Idaho were undertaken to reduce the uncertainty in hydrologic hazard estimates at low annual exceedance probabilities near Bureau of Reclamation high hazard dams. Stratigraphic descriptions, soil profiles and radiocarbon dating of alluvial deposits combined with two-dimensional hydraulic modeling were utilized to refine extreme flood magnitude and frequency estimates on the South Fork Boise River and Middle Fork Boise River.

Results indicate that extreme floods are larger in magnitude on the Middle Fork than on the South Fork Boise River. This conclusion is supported by the historical gaging record and the soil/stratigraphic record. The hydraulic models show that stream terraces along the South Fork Boise River are inundated by approximately half the discharge magnitude than stream terraces of comparable age on the Middle Fork Boise River even though the sites have similar drainage areas. The largest historical floods recorded by those stream gages also show that larger magnitude floods have occurred on the Middle Fork Boise River relative to the South Fork Boise River when comparing unregulated records from stream gages in the basin. For example, the peaks of record on the South Fork Boise River near Lenox, ID and Featherville, ID are either closely matched or exceeded by the peaks for the same date on the Middle Fork Boise River near Twin Springs, ID. Furthermore, the peak of record as well as several other large historical peaks on the Middle Fork Boise River near Twin Springs, ID far exceed any peak discharge recorded on the South Fork Boise River, by about 1.5 to >2 times. We evaluate several factors that could explain the differences, including the position of storm tracks, basin physiography, and basin shape and aspect. The results highlight differences in extreme flood potential between adjacent basins and indicate that the differences in basin response are related to enduring physical characteristics of the basins and are not a product of a biased, short-term systematic record.