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<br />- <br /> <br />0016~4 <br /> <br />e <br /> <br />Software (2001), non-parametric statistics were used for computing IRA parameters. The <br />coefficient of diBpersion (CD) was defined as equal to (75th percentile - 25lh percentile) I <br />50th percentile. To allow comparison of San Juan River results with those from the <br />Colorado and GJieen Riv~chter et at, 1998), the high and low pulse thresholds were set <br />as the median plus or minus 25 percent. The R V A category boundaries were also <br />established as the median plus or minus 25 percent (i.e. between the 25th and 75th <br />percentiles). Hy:lrologic alteration (HA) values were classified as low (-0.33 to +0.33). <br />moderate (-0.34 to -0.67 and +0.34 to +0.67), or high (-0.68 to - 1.00 and +0.68 to + 1.00) <br />following the guidance of Richter et al., 1998. Absolute values were used to calculate all <br />HA means. <br /> <br />~NV\- <br /> <br />f <br /> <br />- <br /> <br />RESULTS <br /> <br />Hydrologic Altl~ration, Post-Navajo Dam Period <br /> <br /> <br />The closure of Navajo Dam and subsequent water operati between 1965 and 1991 <br />resulted in somt: hydrologic changes being observed on e San Juan River (Table 1; <br />Figures 1 to 4). Of the 33 parameters evaluated by the four were found to be highly <br />altered from th(~ pre-development record (December d January median flows, the 90~day <br />minimum flow, and base flow), while nine were m aerately altered and 20 fell into the low <br />alteration category. CD values declined for 15 0 llie 33 parameters. In general, parameters <br />I'? describing low flow conditions were more hi altered than those for high flow, and most <br />) ~~S\were neg:3tive, indicating fewer observ 'ons were falling within the RV A range than <br />; expected. In mc,st cases, flow llgmentatlOn uring typical low flow periods was <br />responsible for the high degree 0 alteration obsetved. HA values for high flow parameters <br />such as the April through July median flows and the maximum. flow values within GrOll #2 JL.. <br />all fell . .. t tion category' verall, the mean HA value for the post-Navajo ~ <br />am period was 0.29, indicating low alteration. ..J: ~ . <br /> <br />~ <br /> <br />Hydrologic AllIteration, SJRIP Period <br /> <br /> <br />Hydrologic altl;:ration during the 1992 to 2002 period was substantially greater than that <br />observed during the post-Navajo Dam period (Table l' . es 1 to 4). f e <br />ame ers eva ua e , mne were oun to e highly altered from the pre-development <br />record, 15 were moderately altered, and nine fell into the low alteration category. CD <br />values declined for 21 of the 33 parameters when compared with the pre-development <br />period and 25 declined when compared with the post-Navajo Dam period. Similar to the <br />post-Navajo pl;:riod~~f"higbly altered SJRIP parameters were those describing low flow <br />conditions (faU and winter median flows, one and three-day minimums, base flow, low <br /> <br />~ <br /> <br />6 <br /> <br />e05-~ 810/600"d 99v-1 <br /> <br />-WOJ~ WdBI:IO EO-BO-l~O <br />