DEVELOPMENT OF THE LOUTSENHIZER DRAINAGE BASIN AND ASSOCIATED COLLAPSIBLE SOILS, MONTROSE AND DELTA COUNTIES, WESTERN COLORADO

The Loutsenhizer Arroyo and related tributaries are part of a large, northwest-trending drainage basin that extends for approximately 20 km between Montrose and Delta, west-central Colorado. At least three distinct alluvial deposits are found in the drainage basin; the most extensive unit is a clayey silt- and sand-dominated alluvium with occasional gravel stringers that was derived primarily from the Mancos Shale. This dry, low-density soil deposit originated as muddy debris flows that filled local drainage basins and valleys during the late Pleistocene and Holocene. A bulk 14C sample taken from the base of the debris flow deposit yielded a conventional age of 9,810±60 14C yr. B.P. (2 Sigma, 95% probability = BP 11,300-11,170) suggesting that the onset of deposition roughly coincides with the tail-end of the Younger Dryas (~11,500 yr. B.P.). The same sediment also crosses underneath Montrose Municipal Airport and runs into the town of Montrose along Cedar Creek and Montrose Arroyo. Many pseudokarst land features, such as sinkholes, soil bridges, fissures, pipes, and subsurface voids are associated with this soil unit, which are primarily the result of soil dispersion and corrasion. Avenues of micro-piping also occur upon dissolution of pedogenic gypsum when the normally dry sediments become wetted. The sinkholes are typically connected to vertical subterranean pipes which originated along animal burrows, root casts or cracks and fissures at the ground surface. The dispersion and corrasion propagates horizontally until an outlet is reached, causing the newly formed subterranean channels to widen. Over time, the subterranean pipes and fissures widen and soil bridges develop which collapse and form sinkholes and gullies. These collapse features can be large enough to engulf people, livestock, and farm implements, and can cause major damage to structures as well as making fields unusable for agricultural purposes.