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<br />. Intermittent streams in northwestem Colorado above about 6,000 It elevation have very little evidence
<br />of substantial runoff and are undersized (table 1; figures 5g and 5h). Exceptions indude the Piceance
<br />Creek, Yellow Creek, and other basins having steep hillslopes, sparce vegetation, and easily erodible
<br />soils, A lack of channel development is due to: (1) little snowpack and snowmelt runoff; and (2) these
<br />tributaries are above the elevation for substantial rainfall intensities, amounts, and have a limited areal
<br />extent of rainstorms and thus rainfall runoff. Many tributaries have substantially undersized channels for
<br />the size of the drainage basin. In many tributaries, minimal active-channel development is evident; rather
<br />the valley bottoms are relatively broad and completely covered with native grasses that indicate they are
<br />essentially undisturbed by moving water. That so many undersized channels occur in the northwestem
<br />Colorado, strongly suggest that not only is annual precipitation low and runoff small, but also that intense
<br />rainfall occurs very infrequently and occur over small areas that little runoff is produced.
<br />
<br />Hillslope rilling and gullying are indicators of intense rainfall and hillslope runoff (Costa and Jarrett, 1981;
<br />Jarrett; 1990b). Soils in most parts of the basin, particularly on sparsely vegetated, steep hillslopes and
<br />in streambanks, are very erodible. Basins having steep slopes and little vegetation cover, such as occur
<br />most basins in northwestem Colorado, have a large potential for erosion (Schumm, 1977), Thus, if intense
<br />rainfall has occurred in northwestem Colorado, there would be substantial hillslope erosion evidence.
<br />However, onsite inspection indicates that rills and gullies are small to non-existent in basins above about
<br />6,000 It in northwestem Colorado. Hillsides having bare surfaces and comprised of sand or finer grained
<br />. soils (thus very erodible) and slopes of 45 degrees or less have minimal rill or gully erosion. Exceptions
<br />include the Piceance Creek, Yellow Creek, and other basins having steep hillslopes, sparce vegetation,
<br />and easily erodible soils. For these basins little rainfall can produce rilling and gullies. However, no
<br />hills lopes have gully development similar to basins at lower elevations in eastem Colorado and below
<br />about 6,500 It in the Colorado River Basin in Colorado that are subject to intense rainstorms.
<br />
<br />The landforms and deposits from floods, primarily flood bars having coarse grained material, are
<br />sufficiently well known and documented (Follansbee and Sawyer, 1948; Bonner and Stermitz, 1967;
<br />Snipes and others, 1974; Schwartz and others, 1975; Helleyand laMarche, 1973; McCain et ai, 1979;
<br />Jarrett, 1987, 1990b, 1991; Jarrett and Costa, 1983, 1986, 1988; Undner-Lunsford, 1988; Grimm, 1993;
<br />Levish and others, 1994; Waythomas and Jarrett, 1994; Jarrett and Waythomas, in press; Pruess, 1996)
<br />that if such evidence existed, it would have been recognized in northwestem Colorado, Floods in lower
<br />elevation (below 7,500 ft) streams in Colorado that have a large-diameter sediment supply (as do most of
<br />the streams in northwestem Colorado) have left identifiable flood deposits in stream valleys. These
<br />deposits result from and reflect multiple extreme floods in each basin dating from 10,000 years to at least
<br />100,000 years ago in un glaciated basins (eg" Jarrett and Waythomas, in press).
<br />
<br />There is an absence of paleoflood evidence of substantial out-of-bank flooding in the Elkhead Creek
<br />Basin and other vegetated basins in northwestem Colorado above about 6,000 It (the paleoflood-study
<br />. area covered about 4,200 mi2) at least since post-glacial times or about 10,000 years. Many basins in
<br />northwestem Colorado are unglaciated, thus, the paleoflood record is longer. Significant weight needs to
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