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I I I I I I I I I I I I I I I I I I I Lower Big Dry Creek Hydrologic Study majority of the watershed upstream of the Weld County line has drainage plans that have been conducted in accordance with regional standards and national norms. Regional storm water detention is a key technique implemented in these plans, with over 2,600 acre-feet planned and roughly 1,800 acre-feet estimated to be currently existing. This represents an investment along the order of $3.5 to 4 million in regional facilities. Based on comparison of projected flows in these master plans and the estimated flows during July 2004, it appears that these facilities are significantly reducing peak flows experienced in the lower watershed. In the absence of these facilities, higher peak flows would have been expected in the lower watershed based on peak flows developed in drainage master plans. 8. Given that the watershed is only about 60 percent developed, downstream residents will likely experience increased volumes, frequency and duration of runoff as development continues to oceur. Provided that detention practices are implemented as planned, peak flow rates should be controlled to a significant extent. These controls are expected to be more effective for a wider range of flow events relative to historical practices because the more frequently occurring flows associated with the water quality capture volume are also now controlled:in addition to the 2- to 5-year storm and the 100-year storm, whereas more recent development may not have provided treatment of the water quality capture volume. 9. Big Dry Creek has not had a natural flow regime in about 100 years due to the construction of Standley Lake in the upper watershed. On average, releases from Standley Lake are believed to constitute roughly 30 percent of the flows present in the creek, while wastewater flows are believed to constitute about 40 percent (annual volume) of the flows in the creek. Base flows in the watershed are characterized by relatively low depths, non-flashy nature and relatively low velocities. Based on a cursory review of these characteristics, base flows would not appear to be the driving force in stream channel erosion. However, a more detailed analysis of stream geomorphology would be required to draw this conclusion with certainty, particularly given some recent studies (e.g., Rohrer and Roesner 2004) indicating that low, constant flows can move large quantities of sediment and can undercut and destabilize banks. It can be said, 971-179.092 June 2005 Wright Water Engineers, Inc. Page 5