T262 Geologic Maps, Digital Geologic Maps, and Derivatives from Geologic and Geophysical Maps (Posters)

Wednesday, 12 October 2011
9:00 AM - 6:00 PM
Hall C

MAPPING OF RIVER ALLUVIUM ALONG THE VERDE RIVER AND MAJOR TRIBUTARIES, CENTRAL ARIZONA

Paper number: T262-4
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

In 2010, the Arizona Geological Survey (AZGS) released a series of 1:24,000-scale geologic strip maps showing the extent of Holocene channel and floodplain deposits along the Verde River and five large tributaries in central Arizona. The primary purpose of this mapping was to contribute to the legal differentiation of river subflow from groundwater in surrounding areas, as Arizona courts have established the lateral extent of Holocene river alluvium as a key component in delineating subflow zones. These maps also define the physical architecture and geomorphic framework of the river systems, depicting river and tributary deposits of Holocene to late Pliocene age, Pliocene and older basin filling deposits, and various bedrock units. We employed standard geomorphic and geologic criteria to differentiate and map river and tributary alluvial deposits of different ages. The degree of clast rounding, lithologic diversity, and landform/terrace slope were used to differentiate river from piedmont alluvium.

The through-flowing Verde River developed several million years ago, linking several closed or partially integrated basins. When the Verde River became integrated, it began to downcut along its entire course, and this in turn drove deep incision along all of its major tributaries. The maximum amount of incision has probably occurred along the middle Verde River in Verde Valley, where the river has incised at least 300 m below the former valley floor since the late Pliocene. High standing relict terrace deposits all along the Verde River record former river levels through the Pleistocene. Because the Verde River and its tributaries have incised deeply into both bedrock and less-resistant sedimentary basins, the lateral extent of Holocene river alluvium varies dramatically from less than 30 m to over 1 km across. Within this corridor, we mapped up to 6 Holocene units, including active channels, bedrock lined active channels, flood channels, and several levels of terraces. The extent of active channels, flood channels and low bars change with virtually every large flood. Young terraces are more stable, and typically are subject to shallow inundation in floods. Dates from archaeological sites and paleoflood studies suggest that nearly all of the Holocene deposits along the river are less than 4 ka.