9422 - Laserfiche WebLink

Home Browse Search

Changes, which have occurred in the magnitude, frequency, timing, and duration of discharges, are discussed in the hydrology section of the accompanying technical report. The four Green River bottomland sites, all of which are within twenty miles of each other (Figure 1), are also in proximity of the razorback spawning habitat near the Jensen gage. Figure 1 shows the location of the Gunnison and Colorado River sites. Under existing conditions, two. of the four sites act as depressions that are inundated by high runoff flows and retain water after peak flows recede. The other two act as terraces. The recovery program defines a terrace as an area that is inundated by high runoff flows that completely drain back to the river when the flood flows recede. The topographic maps for each site are appended. A site by site description of each bottomland follows: GRSN303W Bottomland -The furthest upstream site, the SN Bottomland, is located just below river mile 303 (303 river miles upstream of the confluence with the Colorado River), approximately one mile upstream of the Highway 40 Jensen Bridge along the west bank (see figure 3). The SN bottomland is a wetland area on property referred to as the Stuntz Valley Ranch. The floodable area is a depression that holds water year round. The topographic survey was conducted in October 1997 indicated that 100 acres of floodable acreage exists. At that time, there were approximately 40 acres of standing water. The wetland area has seepage connections to the river and is also fed by small upland drainages that feed the depression. The wetland vegetation is comprised mostly of tall reeds (up to 8') and cat tails where water stands year round, with shorter grasses and weeds on the outer edges of the wetland. A high natural levee lined with mature cottonwoods separates the wetland and river and prevents frequent, widespread overbank flooding. The riverbank is however, lower in elevation at the downstream end of the wetland and allows overbank flooding when flows exceed 17,000 cfs in the Green River. A lateral man made ditch connects to the river at this location. The lower end is the most logical location for any flooding enhancement opportunities (this will be discussed later in this report). The floodplain hydraulics of this site closely resembles those of the Bonanza Bridge and Stirrup Bottomlands (FLO, 1997). Access to the site is fair. County road 149 (from Jensen) leads to Mr. Snow's house, and an access road from Mr. Snow's house leads to the wetland. However, the road is inundated at high river flows, limiting access to the river. GRIM302.5W Bottomland -Located almost immediately below the GRSN303W Bottomland along the west riverbank, at river mile 302.5, the IM Bottomland provides up to 13 acres of floodplain terrace. The IM property is a narrow strip of land that is bordered to the north by the MU property and the west by the BR property. The area listed is the floodable area on the IM property only, a small amount of flooding extends upstream to the MU property at high flows (see figure 3). The owner of the property between the SN and IM property refused to allow field crews on his property, so contour information was assumed to be continuous from the up and downstream mapping. The vegetation on the BR Property mostly consists of grasses. The property had very little standing water in October. Some groundwater connections from the adjacent wetland exist, but in general, the site only receives water when overbank flooding occurs after flows exceed 20,900 cfs. All flood flows return to the river after the flood stage drops. Access to the site is poor, it involves crossing adjacent properties or the use of a boat. ' Note that all floodable acreage quantities listed in the descriptions refer to inundation areas corresponding to 26,400 cfs, the 10-year return period based on post-1963 hydrology of flow in the Green River. • 1-2