Hazards and Flood History in the San Miguel River Valley, Colorado:
Background for Rehabilitation From Past Disturbances
Figure 1. Digital elevation model of the San Miguel River basin showing the location of project study areas. Telluride, near the upper end of the basin, is an historic mining town that has become a major ski resort, whereas Uravan, near the lower end of the basin, is a haunting reminder of the Atomic Age that is now only a name on the map.
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Flowing through San Miguel and Montrose, Counties, in western Colorado, the San Miguel River is one of the most pristine in the Southern Rocky Mountains. However, the river basin has experienced localized, intensive development from gold mining and urbanization in its headwaters (Telluride-Ophir-Placerville area) and uranium mining and milling in the downstream reaches, an area that includes a Superfund site. USGS scientists are (1) evaluating how natural processes and human development have altered the river channel, (2) delineating areas of landslide and debris-flow hazards, and (3) reconstructing the basin's flood history. Such work provides scientific information and interpretations critical to developing strategies for rehabilitation of disturbed areas within the river basin that is planned by federal and local governments, along with non-governmental conservation and citizens' groups.
Figure 2. View of the San Miguel River valley looking southwest from a location on Highway 145 east of Norwood. Like many rivers on the Colorado Plateau, the San Miguel River is in a canyon. The canyon is 650-1000 ft deep in most places and as much as 2000 ft deep just west of Placerville. The valley floor is generally narrow, ranging in width from less than 100 ft to nearly 900 ft.
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Aerial photography obtained at intervals over the past half century were used to identify reaches where the river has been impacted by human activities. The reaches were sorted into three categories: (1) those that probably can not be rehabilitated even with great expenditures of time and money, (2) those that will rehabilitate on their own in a few decades or less, and (3) those that can be rehabilitated, but only if help is provided. Understanding how the San Miguel River has functioned in recent years, as well as in the recent geologic past, is a requisite for developing rehabilitation strategies. Thus, project goals include documenting the timing, magnitude, and rate of channel migration using aerial photography, and reconstructing river history by means of stratigraphic studies and radiocarbon dating.
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Figure 3. Surficial geologic map of the study reach near Uravan. Coarse debris-fan deposits near the southwest corner of the map constrain lateral movement of the San Miguel River and inhibit downcutting. Consequently, just upstream from the debris fan, aggradation is widespread and the channel has migrated laterally to such an extent that more than half of the valley-floor deposits have been reworked in less than 50 years. In contrast, where debris fans project into the valley or the channel is entrenched in resistant bedrock, such as underlies late Pleistocene terrace deposits in the northeastern part of the map area, the position of the channel has changed little during the past 50 years.
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Geologic maps made of selected reaches show that during the past 12,000 years, erosion and valley deepening have been the dominant processes along the San Miguel River. The present-day river is about 12 m (40 ft) below remnants of sand and gravel that were deposited by meltwater on the valley floor that existed when valley glaciers occupied the upper part of the drainage basin. Since the disappearance of those glaciers, valley-side processes and tributary streams have supplied most of the sediment delivered to the river. Much of the sediment from gullies and tributary streams has accumulated in fan-shaped masses where the side-valley channels join the valley floor of the San Miguel River. These fan-shaped masses, called debris fans here because they contain an abundance of debris-flow deposits and coarse rock fragments, make up half of the surficial deposits on the valley floor of some reaches. Where debris-fan deposits extend most of the way across the valley floor, which they commonly do opposite the mouths of the larger side valleys, they act as local base levels because the coarse rock rubble in them armors the river channel and, thus, inhibits incision of the channel. Alluviation and lateral migration of the channel is most pronounced just upstream from what, in effect, are debris-fan dams. In these places, late Holocene alluvium covers most of the valley floor and the rate of lateral channel migration is such that the river has reworked more than half of the valley-floor alluvium within the past 50 years. More common, however, are reaches where the river weaves its way around and between a succession of debris fans, or is largely constrained by valley walls entrenched in bedrock, and has remained in essentially the same location for the past half-century.
 
Figure 4. A. View of a debris-flow that partially buried a residence near Sawpit following a severe thunderstorm on Friday July 30, 1999. B. Debris-flow deposit inside a second Sawpit residence. As described by Linda Luther in the San Miguel Watershed Newsletter, debris flows "closed Highway 145 in various places trapping hundreds of cars in the canyon during Friday's rush hour exodus from Telluride." "The rising waters were Sawpit red [because of sediment from redbeds of Permian and Triassic age] heavily laden with silt, choked with timbers, a moving mass of solids. The sound was that of a highly amplified concrete mixer as rocks, boulders, and timbers churned and ground their way past." (Photos by Linda Luther, Placerville, CO).
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U.S. Department of the Interior
U.S. Geological Survey
This page is <http://gec.cr.usgs.gov/info/sw/sanmiguel/index.html>
Maintained by Randy Schumann
Last Modified Tuesday, 20-Jun-2000 09:03:19 MDT
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