Stream: San Miguel River Executive Summary

Home Browse Search

Miguel River there is a USGS gage record of discharge on the creek. However, the gage station is upstream from the LT. The USGS gage is SAN MIGUEL RIVER AT URAVAN, CO (USGS 09177000); it has a period of record (POR), of 42 years collected between 1953 and 2007. The gale is at an elevation of 5,000 ft above mean sea level (amsl) and has a drainage area of 1499 mi . The hydrograph (plot of discharge over time) produced from this gage includes the consumptive uses of numerous diversions in the basin above the gage. However, the existence of these diversions is not a major limitation upon the use of the data from the gage. To make the measured data transferable to San Miguel River above the LT, the consumptive portions of these diversions were added back to the measured hydrograph. The resulting "adjusted" hydrograph could then be used on San Miguel River above the LT by multiplying the "adjusted" gage discharge values by an area ratio; specifically, the area of San Miguel River above the LT (1557.17 mi) to San Miguel River at Uravan (1499 mi) . Next, the resulting proportioned "adjusted" hydrograph was itself "adjusted" (decreased) to reflect depletions on San Miguel River above the LT resulting from upstream consumptive irrigation uses. The final hydrograph thus represents a distribution of flow over time that has been reduced to reflect existing human uses. (The following discussion is based upon the US Geological Survey's Techniques of Water - Resources Investigations Series, Book 4: Hydrologic Analysis and Interpretation, Chapter A3: Statistical Methods in Water Resources (Chapter 3: Describing Uncertainty) by D.R. Helsel and R. M. Hirsch. This technical reference provides the scientific background and guidance important to the systematic interpretation of hydrologic data. The document is available online and is a valuable aid to understanding and interpreting the analyses described here.) The next step in producing a representation of the discharge at San Miguel River above LT is to compute the Geometric Mean of the area- prorated "adjusted" data values from the San Miguel River at Uravan hydrograph. This step is of value because of the inherent statistical weaknesses found in any collection of data intended to measure natural stream discharge. Without getting into the details of statistical theory, it is worth noting that a set of discharge measurements is inherently inaccurate, no matter how well collected, due to the difficulties attendant to data collection, especially hydrologic data. To give deference to this fact and to increase the value of the hydrograph product of this analysis, the Geometric Means of the data were computed and plotted along with the 95% Confidence Intervals about the data. The resultant hydrograph, including recommended Instream Flow values, is displayed in Figure 1. -8-