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I I I I I I I I I I I I I I I I I I I Wetland areas occur intermittently throughout this study area. These wetlands provide a benefit to surface-water quality by capturing sediment and other contaminants carried in stonnwaters. Figure 4 depicts the type and location of the wetlands delineated for this watershed study area. 2.4 FLOOD MITIGATION MEASURES Numerous irrigation ditches transverse this basin generally from west to east. In many cases, the surface runoff in Bullhead Gulch and its tributaries is intercepted by these ditches and carried to a nearby reservoir. The wetlands located along the irrigation ditches and the edges of the reservoirs generally help to improve water quality. Conversely, the wasteways and associated return flows from irrigation ditches and reservoirs can cause degradation in water quality and increased soil erosion, due to the point source discharge onto unprotected areas. A number of irrigation ditches cross the channels and floodplains within the study area. Separation of irrigation and flood flows was assumed for all irrigation structures, In addition, several small privately owned irrigation and water supply facilities are located within the study area. These facilities were assumed ineffective for flood control purposes and inadvertent flood routing was not considered in the hydrologic analysis of the respective drainage basins. 2.3 FLOOD HISTORY The past flooding which has occurred in this watershed is not well documented, due to the predominately rural nature of the area. However, long time residents have described major flooding along Bullhead Gulch in the past. According to local residents, significant floods occurred three times over the last 60 years, at roughly 20-year intervals. The depth of flooding over the intersection of Isabelle Road and U.S, Highway 287 was estimated at 3 feet during one storm event. This same storm was reported to have moved an existing house off of its foundation. The reach of Bullhead Gulch between Isabelle Road and the Leyner-Cottonwood Ditch is currently a gently sloping, broad floodplain area with no visible low flow channel. Prior to construction of the Leyner-Cottonwood Ditch, Bullhead Gulch had naturally eroded a channel 15- to 20-feet deep. This natural channel has since been artificially filled and is currently the site of a nursery, greenhouses, and agricultural land. FLOOD HAZARD AREA DELlNEA nON BULLHEAD GULCH AND TRIBUTARIES FINAL 7131191 5