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Mary L. Gitlam, Ph.D. <br />Consulting Geologist <br />October 5, 2005 <br />Mr. David Dillon <br />899 CR252 <br />Durango, CO 81301 <br />115 Meadow Road East <br />Durango, CO 81301 <br />970-259-0966 0 970-259-6064 fax <br />gillam@rnri.net <br />Received <br />OCT 1 7 2005 <br />Cur, nco Fild Ot`ce <br />Divleion rt?.~ir~~.iels /~ Geology <br />Re: River Incision at Dalton Gravel Pit, 1976-2004 Dai-{c~ (,aid ~C~l~le <br />Nl- i2R-7-o~'t <br />Dear Mr. Dillon: <br />I recently prepared this diagram which shows approximate thalweg incision at the <br />Dalton and Hermosa Meadows gravel pits since 1976. <br />This diagram was constructed as follows. Elevations that were originally reported <br />with respect to the NGVD 29 geodetic datum were adjusted to the NAVD 88 geodetic <br />datum by adding 3.44 ft, as determined by Marcia Talvitie of Four Corners Materials. <br />The 1976 data represent the presumed year of field work performed for the USAGE flood <br />study that was published in 1977. Available information suggests that the streambed <br />elevations shown in the 1977 report may have been approximately 2 ft higher than the <br />thalweg at that time, so I have subtracted 2 ft to obtain an estimated thalweg elevations <br />for 1976. This adjustment allows the 1976 estimated thalweg profile and later surveyed <br />thalweg profiles to be compared more accurately. <br />At the Dalton Pit, it appears that the thalweg profile incised approximately 9 to 15 <br />fl from 1976 to 2004. From 1996-1997 to 2004 the thalweg profile incised an average of <br />2.3 ft at the 16 measured cross sections. <br />Please feel free to call me if you have any questions about this analysis. <br />Sincerely, <br />~k1~ ~'~ .~((~c.>~ <br />Mary L. Gillam <br />cc: <br />Ms. Kara Hellige, U.S. Army Corps of Engineers <br />Mr. Wallace Erickson, Colorado Division of Minerals and Geology <br />Ms. Nancy Lauro, La Plata County Department of Community Development <br />Mr. Bill Simon, Animas River Stakeholders <br />