LANDSLIDES IN THE UNCOMPAHGRE VALLEY OF WESTERN COLORADO AND IMPACTS TO MIXED AGRICULTURAL AND RESIDENTIAL LAND USAGE

The Uncompahgre River Valley is the southernmost expression of a broad erosional landform along the north and east side of the Uncompahgre Plateau that marks the surface exposure of the Mancos Shale in western Colorado. From Montrose to Delta, this broad 18-km wide Mancos valley is floored by the lower Uncompahgre River. It is characterized by “adobe” hills of exposed Mancos Shale; sequences of terraces from Pleistocene glacio-fluvial runoff from the San Juan Mountains; and, to a lesser extent, broad Pleistocene alluvial fans from the Uncompahgre Plateau. The terrace and alluvial fan remnants, colloquially called “mesas”, are typically capped by highly permeable, variable-thickness, sandy and cobbly gravel. Ground water percolates through the gravel, perches on the contact with fractured and weathered Mancos Shale, and moves laterally to the mesa edges. The steeper slopes of the mesa edge, the weakened state of the underlying shale, and water naturally cause landslides that typically rim the individual mesas. Land use on the mesa tops has been agriculture since the Uncompahgre Valley irrigation network was established in the early 1900s. Flood irrigation is most commonly used. The frequency of landsliding has increased since irrigation began, but has not been a significant concern with present-day land use since the steeper mesa slopes are not suitable for irrigated agriculture. As development pressures mount in the Uncompahgre Valley, “view” lots, either on the steeper slopes or near the mesa edges, are of premium value. Many of these areas are marginally stable in the short term, but inadvertent and/or heavy irrigation on adjacent farmland may destabilize areas of new residential development in the future. This situation can create a land-use regulation quagmire for local planning agencies that need to balance the demands of developers with agricultural irrigation practices.