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

Home Browse Search

precipitation event is 1 inch in 24 hours. It is unlikely that a storm of this magnitude would extend over the entire East Salt Creek drainage area. Runoff generated from such an event is estimated at approximately 0.03 cfs per acre and drainage basins within the project impacted area could potentially produce 195 cfs. The proposed dry wash crossings are located in the upper reaches of drainage basins. The dry wash crossing points range from approximately 0.5 miles to 20 plus miles (river miles) away from East Salt Wash (the nearest RPW) and an additional 5 to 10 miles from the nearest TNW, the Colorado River. Individual drainages average 38.55 acres, the smallest being 02 acres and the largest being 951 acres. Some of the smaller drainage basin areas are contained within boundaries of larger drainage basins. The total area of all individual drainages represent less than 0.04% of the total drainage area in the East Salt Creek drainage basin and less than 0.02% of the total area of the Salt Creek Watershed contributing to the nearest TNW, Colorado River. Drainage information is contained in Table 2. Washes in drainage basins with areas of 35 acres or larger typically had channels with a predominantly gravel substrate with some sand and cobble. These channels were evaluated throughout their individual lengths to the point where the OHWM was no longer clear and distinct and surface water connectivity was no longer evident. The largest drainage basin, 951 acres, within the project impact area contributes to crossing point W 100. The wash disperses 1.5 miles below crossing point W 100 forming an alluvial fan. Weak indicators of OHWM and perennial and annual vegetation growing in the channel bottoms were observed at the time of survey. Changes in the channel as it flows downstream are depicted in a sequence of pictures provided in Appendix B. The photos illustrate changes in the OHWM and lack of surface water connectivity. Similar trends are present in the large drainages to the south below grouped points W080, W075, W041, and W035. Chemical No water was present in washes during the time of survey. Potential railroad crossing points are typically located in upper reaches of the individual drainage basins and even in high intensity localized precipitation events are not considered capable of contributing significant sediment and nutrients or transporting pollutants to down stream RPWs. The Salt Creek watershed extends 30 aerial miles from the Colorado River to the Book Cliffs. Elevation at the base of the Book Cliffs is 5,486 ft; elevations in the Book Cliff portion of the watershed exceed 8,000 ft. The range of elevation in the project area is 4,400 to 5,200 ft. Spring runoff events are associated with precipitation and snow melt from the higher elevations. Snow accumulation below 5,500 ft. is minimal and seldom remains on the ground for more than a few days (NWCC 2007). Chemical transport functions of the drainages is most likely insignificant; however, during severe wide spread precipitation events the washes could connect with East Salt Creek (RPW) and transport sediment and potential pollutants downstream. Naturally occurring selenium in Mancos shale could be transported during these events. WestWater Engineering Page 4 of 24 COE Jurisdictional Determination Request DBMS 393