Episodic sediment delivery and landscape connectivity in the Mancos Shale badlands and Fremont River system, Utah, USA

Abstract The Fremont River drains about 1000 km 2 of Mancos Shale badlands, which provide a large percentage of the total sediment load of its middle and lower reaches. Factors controlling sediment movement include: weathering that produces thin paralithic soils, mass movement events that move the soil onto locations susceptible to fluvial transport, intense precipitation events that move the sediment along rills and across local pediments, and finally Fremont River floods that move the sediment to the main-stem Colorado River. A forty-year erosion-pin study has shown that down-slope creep moves the weathered shale crust an average of 5.9 cm/yr. Weather records and our monitoring show that wet winters add large slab failures and mudflows. Recent sediment-trap studies show that about 95% of sediment movement across pediments is accomplished by high-intensity summer convective storms. Between 1890 and 1910, a series of large autumn floods swept down the Fremont River, eroding its floodplain and transforming it from a narrow and meandering channel to a broad, braided one. Beginning about 1940, the Fremont's channel began to narrow. Sequential aerial photos and cross-sections suggest that floodplain construction since about 1966 has stored about 4000 to 8000 m 3 of sediment per kilometer per year. These data suggest that it will take two centuries to restore the floodplain to its pre-1890 condition, which is in line with geologic studies elsewhere on the Colorado Plateau. The various landscape elements of slope, pediment, and floodplain are semi-independent actors in sediment delivery, each with its own style. Accelerated mass movement on the slopes has an approximate 20-year recurrence. Sediment movement from slope across pediments to master stream is episodic and recurs more frequently. The slope-to-pediment portion of the system appears well connected. However, sediment transport through the floodplain is not well connected in the decadal time scale, but increases in the century and millennial time scales, and changes over time depending on the cycle of arroyo cutting and filling.

[1]  Robert S. Anderson,et al.  Controls on flash flood magnitude and hydrograph shape, Upper Blue Hills badlands, Utah , 1997 .

[2]  A. Harvey.  Effective timescales of coupling within fluvial systems , 2002 .

[3]  R. Cooke,et al.  Arroyos and Environmental Change in the American Southwest , 1976 .

[4]  Mass Movement of Mancos Shale Crust Near Caineville, Utah: A 30‐year Record , 1997 .

[5]  S. A. Schumm,et al.  Seasonal variation of infiltration capacity and runoff on hillslopes in western Colorado , 1963 .

[6]  D. E. Burkham.  Effects of changes in an alluvial channel on the timing, magnitude, and transformation of flood waves, southeastern Arizona , 1976 .

[7]  D. Gutzler,et al.  Geomorphic response to seasonal variations in rainfall in the Southwest United States , 2004 .

[8]  W. Graf.  Late Holocene sediment storage in canyons of the Colorado Plateau , 1987 .

[9]  A. Goudie.  Encyclopedia of Geomorphology , 2003 .

[10]  D. E. Burkham.  Hydraulic effects of changes in bottom land vegetation on three major floods, Gila River, in southeastern Arizona , 1976 .

[11]  A. Godfrey.  Wind erosion of Mancos Shale badland ridges by sudden drops in pressure , 1997 .

[12]  R. Anderson,et al.  Cosmogenic dating of fluvial terraces, Fremont River, Utah , 1997 .

[13]  R. Hereford.  Valley-fill alluviation during the Little Ice Age (ca. A.D. 1400–1880), Paria River basin and southern Colorado Plateau, United States , 2002 .

[14]  R. Hereford.  Modern Alluvial History of the Paria Rver Drainage Basin, Southern Utah , 1986, Quaternary Research.

[15]  A. M. Harvey,et al.  Coupling between hillslope gully systems and stream channels in the Howgill Fells, northwest England: temporal implications/Le couplage des systèmes de ravins et des lits fluviaux dans les Howgill Fells, nord-ouest de l'Angleterre : signification temporelle , 1997 .

[16]  Kenneth G. V. Smith.  EROSIONAL PROCESSES AND LANDFORMS IN BADLANDS NATIONAL MONUMENT, SOUTH DAKOTA , 1958 .

[17]  B. Hart,et al.  Porosity and pressure: Role of compaction disequilibrium in the development of geopressures in a Gulf Coast Pleistocene basin , 1995 .

[18]  S. Schumm.  EVOLUTION OF DRAINAGE SYSTEMS AND SLOPES IN BADLANDS AT PERTH AMBOY, NEW JERSEY , 1956 .

[19]  W. L. Stokes.  Geology of Utah , 1986 .

[20]  A. Harvey,et al.  Coupling between hillslopes and channels in upland fluvial systems: implications for landscape sensitivity, illustrated from the Howgill Fells, northwest England , 2001 .

[21]  W. M. Hess,et al.  Natural History of the Colorado Plateau and Great Basin , 1994 .

[22]  B. Everitt.  Chronology of the spread of tamarisk in the central Rio Grande , 1998, Wetlands.

[23]  R. Bryan,et al.  Badland geomorphology and piping , 1984 .

[24]  R. Hereford,et al.  Channel evolution and hydrologic variations in the Colorado River basin: Factors influencing sediment and salt loads , 1991 .

[25]  A. Parsons,et al.  Geomorphology of Desert Environments , 1994 .

[26]  B. Everitt.  Channel responses to declining flow on the Rio Grande between Ft. Quitman and Presidio, Texas , 1993 .

[27]  Ralph L. Miller,et al.  Geology and geography of the Henry Mountains region, Utah , 1953 .

[28]  S. Schumm.  Arroyos and Environmental Change in the American SouthwestR.U. Cooke and R.W. Reeves, 1976. Oxford Research Studies in Geography. Oxford University Pres, Oxford, 213 pp. £8.50 , 1977 .

[29]  R. Webb,et al.  Historic variation of warm-season rainfall, Southern Colorado Plateau, Southwestern U.S.A. , 1992 .