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for a long period of time (the longer the better) including wet and dry periods. In the case of <br />Animas River - Lower there is a USGS gage record of discharge on the creek. However, the <br />gage station is upstream from the LT. The USGS gage is ANIMAS RIVER AT <br />HOWARDSVILLE, CO (USGS 09357500); it has a period of record. (POR) of 47 years <br />collected between 1935 and 1982. The gage is at an elevation of 9,616.98 R above mean sea <br />level (amsl) and has a drainage area of 55.9 mil. The hydrograph (plot of discharge over time) <br />produced from this gage includes the consumptive uses of several diversions in the basin above <br />the gage. However, the existence of these diversions is not a major limitation upon the use of the <br />data from the gage. To make the measured data transferable to Animas River - Lower above the <br />LT, the consumptive portions of these diversions were added back to the measured hydrograph. <br />The resulting "adjusted" hydrograph could then be used on Animas River = Lower above the LT <br />by multiplying the "adjusted" gage discharge values by an area ratio; specifically, the area of <br />Animas River - Lower above the LT (59.85 mi) to Animas River at Howardsville (55.9 mi). <br />Next, the resulting proportioned "adjusted" hydrograph was itself "adjusted" (decreased) to <br />reflect depletions on Animas River - Lower above the LT resulting from upstream consumptive <br />irrigation uses. The final hydrograph thus represents a distribution of flow over time that has <br />been reduced to reflect existing human uses. <br />(The Following discussion is based upon the US Geological Survey's Techniques of Water - <br />Resources Investigations Series, Book 4: Hydrologic Analysis and Interpretation, Chapter A3: <br />Statistical Methods in Water Resources (Chapter 3: Describing Uncertainty) by D.R. Helsel and <br />R. M. Hirsch. This technical reference provides the scientific background and guidance <br />important to the systematic interpretation of hydrologic data. The document is available online <br />and is a valuable aid to understanding and interpreting the analyses described here.) <br />The next step in producing a representation of the discharge at Animas River - Lower is to <br />compute the Geometric Mean of the area- prorated "adjusted" data values from the Animas River <br />at Howardsville hydrograph. This step is of value because of the inherent statistical weaknesses <br />found in any collection of data intended to measure natural stream discharge. Without getting <br />into the details of statistical theory, it is worth noting that a set of discharge measurements is <br />inherently inaccurate, no matter how well collected, due to the difficulties attendant to data <br />collection, especially hydrologic data. To give deference to this fact and to increase the value <br />of the hydrograph product of this analysis, the Geometric Means of the data were computed and <br />plotted along with the 95% Confidence Intervals about the data. The resultant hydrograph, <br />including recommended Instream Flow values, is displayed in Figure 1 with the data displayed in <br />Table 2. <br />-5- <br />