HOLOCENE MIGRATION OF BEAR RIVER, IDAHO

The low-lying, fault-bounded region between Bear Lake and Bear River is filled with marshes and abandoned meandering channels. Surficial sediments record Holocene shifts of Bear River channels and of north-flowing axial channels draining western tributary streams and Bear Lake itself. Aerial photographs show that several abandoned river courses lie south of modern Bear River. Radiocarbon ages and stratigraphy in canal cuts and auger holes show that the oldest course, BR1, flowed south toward Bear Lake ca. 8.6-7.9 cal ka, and suggest that this river course may have caused the coeval rise of Bear Lake. BR2 and BR3 lie between BR1 and the present river, and were tributary to north-flowing axial drainages that also carried the flow of west-side creeks and probably discharge from Bear Lake. Orientations of BR2 and BR3 and a few ¹⁴C ages show that Bear River progressively shifted northward and eventually away from the axial drainage. The initial shift north from BR1 occurred no later than 5.8 cal ka, the shift from BR2 to BR3 occurred by about 2.5 cal ka, and the shift to the modern course of Bear River probably occurred after about 1.3 cal ka. The history of the north-flowing axial channels is not well defined, but several ages from this study and previous work of J. McCalpin indicate that one north-flowing course was established by 8.5 ka and was abandoned by about 7.4-6.7 cal ka. The BR2 and BR3 river courses merged with a younger north-trending channel to the east.

Several factors, including faulting, location of the outwash fan, and fluvial geomorphic processes, probably interacted to produce Holocene changes in channel position. Course BR1 may simply represent an avulsion of Bear River as it migrated across its post-glacial outwash fan, and this diversion resulted in the river merging with the lake, causing the lake to rise. The location of the north-flowing west-side axial channel was probably controlled by the presence of the outwash fan as well as active graben structures along the West Bear Lake fault zone. The reasons for the northward shift of Bear River since ca. 8 cal ka may include (1) avulsion, (2) inactivity of the West Bear Lake fault zone since the early Holocene, and (3) decreased flow in Bear River out-competed by northward groundwater flow out of Bear Lake.