Upper Colorado River Basin River Thalweg

The Southern Rockies LCC is home to narrowleaf cottonwood (Populus angustifolia), common at elevations above 1800 m, and Fremont cottonwood [a common name regionally attached to the ecologically very similar Populus fremontii subsp. fremontii S. Watson and P. deltoides subsp. wislizenii (S. Watson) Eckenwalder, as well as their intergrades], which is typically found at elevations below 1800 m. This geographical information system (GIS) contains the data sets used in an assessment of the amount and character of native Fremont cottonwood forest remaining on the mainstem floodplains in 26 subbasins in the Upper Colorado River Basin (UCRB), published in 2007 (Andersen, D.C., D.J. Cooper, and K. Northcott. 2007. Dams, floodplain land use, and riparian forest conservation in the semiarid Upper Colorado River Basin, USA. Environmental Management 40(3): 453-475). That assessment involved interpretation of black-and-white Digital Orthophoto Quarter Quadrangles, with a 1-m pixel resolution, generated from imagery collected in 1988 or later. The study produced a very large amount of geographical information, including data on land cover and land use for ~60,000 ha of floodplain along 2600 km of stream channel. The vast majority of the data (i.e., 24 of the 26 subbasins assessed) was from locations within the Southern Rockies LCC, and the remainder was from immediately adjacent locations. Seven land cover types were identified for all floodplain areas below an elevation of 1800 m (used as the upper distributional limit of Fremont cottonwood): cottonwood forest, agriculture, industry, road, urban, water and other vegetation. The cottonwood forest polygons were each assigned to a canopy density class. The original assessment found that although more than half of the interpretable floodplain area in the images (i.e., non-canyon mainstem floodplain area in the 1990s) was cottonwood forest, nearly 70% of that forest area featured a sparse canopy (≤5% cover), strongly suggesting it was made up solely of old trees. Stands with >50% canopy, typical of young trees, occupied <1% of the floodplain. These patterns suggest that, on a regional basis, there is insufficient recruitment to replace stands of old trees and that a major reduction in forest extent will occur when the old trees die. The assessment also suggested that the increase in Fremont cottonwood abundance noted on some streams and rivers in the American Southwest is probably limited to the extreme southern portion of the Southern Rockies LCC and on streams where the largest floods were historically associated with monsoon or winter rains rather than spring snowmelt. This feature class represents the thalweg, the approximate location of the deepest part of the stream channel.