2008-02-22_APPLICATION CORRESPONDENCE - C2008086 (42)

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Points WOl 1-W019 originate from an old irrigation ditch constructed on the upland terrace that runs along the west side of Mack Mesa. The ditch is no longer functional and has numerous failures along its length. Eroded gullies have developed below many of the failures in the ditch and were not considered to be jurisdictional tributaries (COE 2007a). Points V001-V060 are believed to be non jurisdictional due to lack of OHWM. These drainages also lacked evidence of flow and contained perennial and annual vegetation in the drainage bottoms, absence of evidence of flow such as shelving and detritus buildup, and lack of connectivity to other WOUS. Points W002-Wll1B are drainages that showed some evidence of an OHWM. The OHWMs within these washes were inconsistent and lack continuity in their flow path to RPW East Salt Creek. These drainages were further evaluated for their potential to significantly alter the physical, chemical, and biological properties of down stream TNW in a significant nexus evaluation. Photos representing typical washes and drainage basins within the project area are in Appendix A -they are labeled by crossing points in Table 2 and mapped in Figures 2 and 3. Significant Nexus Evaluation Physical These dry washes are believed to be non-RPW's with no abutting or adjacent wetlands and are contained within the East Salt Creek drainage. The East Salt Creek drainage covers approximately 225 square miles of which approximately 151 square miles are part of the Book Cliffs geographic area to the north of the project area. The proposed project utilizes approximately 16 of the remaining 74 square miles of the lower East Salt Creels drainage. The Book Cliffs provide snow melt and spring runoff from elevations up to 8500 ft. The lower valley of East Salt drainage receives 9.18 inches of precipitation annually with most of the volume of flow in the washes associated with precipitation events between the months of April and October (NWCC 2007). Typically high flow volumes in neighboring drainages are associated with October precipitation events; however, the spring runoff month of May contributed the highest average flow volume in East Salt Creek over the gauging period of record. Spring flows are related to snow melt from the 14 miles the East Salt Creek drainage extends into the Book Cliffs. Peak flows at the gauging station in East Salt Creek averaged 30 cfs and are typically sustained for an average of 15 days during the months of peak runoff. Intermittent flows in East Salt Creels outside of the peak runoff months average less than 5 cfs. The USGS gauging station 9163310 in East Salt Wash (recording period 1973-1982), is located 4.5 straight miles and 7.92 river miles upstream from the confluence of the East and West Salt Wash (RPW) and measured run off for 197 square miles of drainage area (USGS 2007). Discharges from storm events are localized into small drainages and are the result of fast moving microburst storms (NWCC 2007). Spatial storm variation can produce runoff in one wash and none in another; variation of precipitation can be as much as 0.4 inches between small drainages within 0.5 linear miles of each other, resulting in intermittent and inconsistent surface water connections between sub-basins and the nearest RPW (USGS 1956-1972). A 2-year WestWater Engineering Page 3 of 24 COE Jurisdictional Determination Request DBMS 392