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ISMP backwater sampling evaluation investigators (e.g., McAda et al. 1994-1999). This was done to determine if abundance levels of those five species in 1997 and 1998 were abnormal relative to historical levels and to understand if any long-term trends were evident in the ISMP data. RESULTS 1997 Sampling A total of 21 backwaters was sampled in the Grand Valley reach of the Colorado River from 9 September to 11 November 1997. The 21 backwaters represented nearly every accessible backwater in the reach that met ISMP criteria and had an estimated surface area of 15,978 in'. Backwater habitat was unevenly distributed among the sub-reaches. Sub-reach 1 contained six backwaters, sub-reach 2 had two backwaters, and sub-reaches 3 and 4 contained six and seven backwaters, respectively. High water (150-300 M3 /sec) in the Colorado River throughout autumn 1997 limited backwater availability to some extent throughout the study area (Fig. 2). Fish abundance in fifteen backwaters was estimated using depletion techniques and fish abundance in the remaining six was estimated using capture-recapture. Sampling effort included 516 seine hauls, 310 minutes of electrofishing, and 24 (24-hour) fyke-net sets. The 42 ISMP seine hauls completed in the 21 backwaters (2 per backwater) encompassed an average of 18 % (5 to 42 %) of the surface area of each backwater (Table 1). Sampling detected a total of five native and 15 introduced fishes and a total of 37,900 fish were sampled from backwaters (Appendix I). Non-native species represented 94.4 % of all fishes captured and sand shiners (41 %), red shiners (25 %) and fathead minnows (19 %) were the most abundant taxa (Fig. 4). Native species represented 5.6 % of the total catch; roundtail -9-