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re-invasion often occurred via larval fish transported in irrigation water. An isolated 21 SA pond had re-invaded with sand shiners when river water was pumped into the pond for irrigation of lawns. Similarly, landowners have reported seeing larval fish in irrigation head boxes. Screen Installation -- Inlet/outlet screens have the potential to control the movement of nonnative fish from floodplain ponds to critical riverine habitat. One screen that required minimal maintenance was installed below a pond outlet pipe (Figure 11). This screen was constructed of an expanded aluminum grate that was covered with window screen and supported on an aluminum frame. The screen was placed horizontally with a slight tilt in which the corner furthest from the outlet water was lower than the other three corners while the corner closest to the outlet water was higher than the other three corners. This tilt allowed debris to be pushed off the edge of the screen by the out- flowing water pressure. This screen effectively removed fish from the out flowing water and required little maintenance. A similarly constructed screen had a box design and was also placed below an outlet pipe (Figure 12). The effectiveness of this screen was not determined. An additional screen design, which was placed on a surface release outlet pipe, fouled frequently in spite of regular cleaning and was eventually removed from the outlet structure (Figure 13). Two screens designed, constructed, and installed by Irrigation Systems Company of Western Colorado employed a gravel filter design (Figure 14) and a self- cleaning, rotating, flat plate screen design (Figure 15). Fouling was a concern with the gravel screen when it was first installed due to excessive filamentous algae and small gravel size (1"). To rectify this situation, the gravel was removed, larger cobbles (3"-4") were installed, and a fence was placed approximately five feet from the screen to reduce fouling of the screen by floating algae. The flat plate screen had been designed for irrigation ditches and used solar energy as a power source. Moving parts included a rotating brush with two heads and a perforated flat-plate screen. Both screens were installed in the summer of 2000 and functioned well in 2001 and 2002. Shearing wedgewire screens (also known as Coanda screens) were installed on the inlet water to Horsethief Canyon State Wildlife Area's (HTSWA) fields, waterfowl ponds, wetland (Figure 16) and endangered fish grow-out ponds (Figure 17). These 34