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<br />.
<br />
<br />1935 Cherry Creek-Hale storm, and the 1965 stonn over Kiowa, Bijou, Plum Creeks and nearby tributaries
<br />(Follansbee and Jones,1922; Follansbee and Hodges, 1925; Follansbee and Spiegel, 1937; Follansbee
<br />and Sawyer, 1948; Matthai, 1969; Snipes and others, 1974; Hansen and others, 1988). These rainstorms
<br />had greater than 8 to 10 in. of rain in a few hours. Transposing distant rainstorms to the mountains of
<br />Colorado appears to vary from the assumption of the PMP methodology to transpose storms short
<br />distances (National Research Council, 1994).
<br />
<br />Numerous reported, major rainstonns or floods at higher elevations in the Colorado Rockies have been
<br />investigated using paleoflood techniques and research of historical documents, These storms were found
<br />not to have occurred, were overestimated or exaggerated rainfall, or were debris flows (Costa and Jarrett,
<br />1981; Jarrett, 1987, 1990b; Jarrett and Crow, 1988; Grimm, 1993; Pruess, 1996; Jarrett and Capesius, in
<br />review). Jarrett (unpublished data, 1988) conducted a paleoflood reconnaissance near Gladstone,
<br />Colorado, because a recorded rainstonn of 8.05 in. in 24 hr on October 4-5,1911 at Gladstone (Hansen
<br />and others, 1988). Gladstone, at an elevation of about 10,500 ft, is a historic mining town located near
<br />Silverton, The 1911 Gladstone rainstonn is one of the key stonns in defining PMP values at higher
<br />elevations in Colorado (Hansen and others, 1988), No paleoflood evidence of flooding was identified in
<br />any streams in the vicinity, and the Gladstone rainstonn of 1911 is highly suspect and believed to have
<br />been measured incorrectly, The October 4-5, 1911 24-hr rainfall of 4.25 in. at Silverton is believed to
<br />reflect maximum rainfall in the San Juan Mountains. This indicated a need for additional investigations of
<br />. the most severe stonn that has occurred in historic times in the San Juan Mountains, and to assess the
<br />validity of the 1911 rainfall amount at Gladstone.
<br />
<br />Pruess (1996) conducted an interdisciplinary (meteorologic, hydrologic, and paleoflOOd) investigation of
<br />the Gladstone area and upper Animas River Basin to assess the hydrometeorology of the San Juan
<br />Mountain region, He did not identify any evidence of substantial out-of-bank flooding in streams in the
<br />Gladstone area associated with the 1911 rainstonn or any other rainstonn. Pruess documented that the
<br />maximum unit discharge for any paleofloods or historical floods above 7,500 ft is less than 100 ft31s1mi2,
<br />which is consistent with studies of Jarrett (1987, 1990a, 1993), Jarrett and Costa (1983,1988), and Grimm
<br />(1993).
<br />
<br />Reported large rainstonns (greater than about 2 in, in 6 hr or less) or unit discharges exceeding about
<br />100 ft31s1mi2 at higher elevations in the Rocky Mountains, because of their significance as large rainstorms
<br />for higher elevations, need to be reviewed (Jarrett, 1990b; Jarrett and Capesius, in review). These
<br />events need to be validated by: (1) comparing data from nearby precipitation gages or bucket surveys;
<br />(2) inspection of downstream streamflow-gaging station data (if available); (3) conducting paleoflood
<br />investigations; and (4) evaluating the accuracy of rainfall, streamflow, and paleoflood data.
<br />
<br />THE STUDY AREA
<br />
<br />.
<br />
<br />Elkhead Creek Basin, which has a drainage area of 205 mi2 at Elkhead Reservoir, is located in
<br />
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