POSSIBLE CONNECTIONS BETWEEN HISTORIC (1880-1940 A.D.) ARROYO CUTTING AND FLOODPLAIN CHANGES ALONG THE COLORADO RIVER IN WESTERN COLORADO

While the historic (ca. 1880-1940 A.D.) arroyo event of the semiarid southwestern U.S. is well documented along tributaries, little data is available on the effects of this event on the trunk streams. Dendrochronology of cottonwood trees on the Colorado River floodplain near Fruita, CO was used to reconstruct the floodplain history, and to determine if any floodplain changes coincided with historic arroyo incision. A total of 16 cores and 42 tree diameter measurements were used to calculate the ages of cottonwoods on different floodplain surfaces.

Three floodplain units were identified based on differences in height above the low-water channel, relief, and vegetation. The modern floodplain (FP1) is within 1-2 m of the low-water channel and is comprised chiefly of loose sand and gravel that forms low-relief ridges and swales. Vegetation is mostly tamarisk and sparse cottonwood trees that range in age from 7 to 44 yrs (1966-2003 A.D.). The second unit is a slightly higher and older floodplain surface (FP2) that rises 1-2 m above FP1 and is covered by loose sand and minor gravel. This surface is also characterized by low-relief ridges and swales. FP2 is the most densely vegetated terrace with large numbers of cottonwood trees as well as tamarisk, shrubs, and grasses. Cottonwoods of FP2 range in age from 46 to 108 yrs (1902-1964 A.D.). The third unit is a low terrace characterized by large cottonwoods (Cottonwood terrace; CT). This surface rises an additional 1-2 m above FP2 and is typically ~3-4 m above the Colorado River. This surface is generally flat and is covered by dry and compacted sand and silt and minor gravel. This surface also contains the least amount of vegetation, primarily low grasses and scattered large cottonwoods with diameters of ~1 m. The age range of the cottonwoods on the CT surface is between 135 and 163 yrs (1847-1875 A.D.).

The gap in time between the maximum age of FP2 (108 yrs; 1902 A.D.), and the minimum age for CT (135 yrs; 1875 A.D.) coincides broadly with historic arroyo cutting events along Colorado Rivers tributaries such as the Little Dolores River, as well as tributaries elsewhere in the Colorado Plateau region. Climatic factors that drove arroyo cutting along tributaries seem to have affected the Colorado River as well.