THE 'BLYTHE ALLUVIUM': HOLOCENE TO MODERN FLOODPLAIN AND CHANNEL DEPOSITS OF THE LOWER COLORADO RIVER, ARIZONA, NEVADA, AND CALIFORNIA

In the lower Colorado River corridor, ancestral Colorado River deposits are collectively referred to as the ‘LOCO group.’ The youngest member of the 'LOCO group', the Blythe alluvium, reflects the last pulse of aggradation on the modern floodplain during the Holocene. Modified by cultivation and regulation, the natural riverscape is increasingly obscured on the historical floodplain. Field-leveling for agriculture smooths the topography while dams and revetments contain and manage the flow. Detailed reconstruction of historical channel change is achieved through mapping of temporal source material, including early topographic maps (1902), cadastral survey plats (1874, 1917), historical aerial photography (1930, 1938, 1948, 1996), and recent satellite imagery (2010-2015).

The Ives Expedition of 1857-1858 produced the first geologic map of the lower Colorado River, showing a narrow, braided channel network as the river emerged from a series of restricted canyons into wide, mountain-flanked valleys. Subsequent to the Ives maps, the history of the river and its deposits is inferred from preserved channel forms and meander scars on aerial photography, combined with other maps constructed prior to regulation. Later imagery clearly shows the effect of regulation; as water and sediment became progressively limited, the once dynamic river became increasingly constrained.

Deposition of the Blythe alluvium followed the global climatic changes of the Pleistocene-Holocene transition and is the last sediment package to record the evolution of the Colorado River system. The subsurface contact between the Blythe alluvium and the other ancestral Colorado River deposits of the 'LOCO group' is uncertain. Carbon dating of wood and lithologic well logs suggest the Blythe alluvium is at least 33 m thick. Integrated mapping of the modern floodplain with pre- and ancestral Colorado River deposits is ongoing and will augment our understanding of the evolution of the lower Colorado River.