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12 I ,I I 'I il I I I I L I ,I 11 I ~." -, ,I :1 ~I ~. ~I I :1 3. HYDROLOGIC AND ECOLOGIC INVESTIGATIONS 3.1 GEOMORPHOLOGICAL ASSESSMENT A geomorphological assessment of the Arkansas River below John Martin Dam was completed in early 1997 by the u.S. Anny Corps of Engineers Waterways Experiment Station. The assessment provides an overview of the geologic, climatologic, and hydrologic forces influencing the Arkansas River during the past century and is included as Appendix C of this report. 3.2 HYDRAULICS AND SEDIMENTATION Historic Hydrology During the first stage of the present study the historic flow data for the Arkansas River below John Martin Dam were evaluated to some extent. This historic infonnation was obtained from the United States Geologic Smvey's (USGS) web site and imported into a spreadsheet program. The data set covers the period from April 1, 1938 through October 31, 1995, although the data have been gathered from three separate gage locations over that time. The three locations are all within two miles of each other with no significant tributary inflow within that distance and are comparable. The data set was divided into two parts; April 1, 1938 to September 30, 1942, and April 1, 1943 to October 31, 1995. Operation of the reservoir officially began March 11, 1943, though water storage actually began in December 1942. The data set was subsequently divided again to look at flows after Water Year 1981, when operational changes went into effect. To compare the pre- and post- dam hydrologic conditions, the daily flow values of the respective parts were sorted by magnitude, and the number of days of each flow was divided by the total number of flow days in the group. lbis generated flow-duration cmves for the periods and allowed comparison. The post-dam data were further divided to include only the dates after Water Year 1981 to reflect current "trans-mountain" operations. The resultant cmves (Figure 6) agree with those in the Geomorphological Assessment which were based on 1913 through 1996 data from the Lamar stream gage. \ The comparisons show a substantial reduction in peak flows following construction-of John Martin Dam. Note that the "Post-Dam" curve includes the "Post '81" data within it. This reduction occurs, expectedly, for rare events, the type the dam was designed to impact. But it also shows a dramatic reduction for the most common events, as well. Further, there has been a substantial reduction in the volume of water moving down the river at this point as evidenced by the decrease in area under both the "Post-Dam" and "Post '81" discharge-duration curves. This reduction is too large to be attributed to the dam and represents a change in river hydrology upstream. The "Post '81" curve shows a departure from the "Post-Dam" curve primarily between the 30% and 65% abscissa values. This apparently depicts the change in operations associated with trans-mountain diversions, but could be influenced by the smaller