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precipitation years), when even 1,240 cfs would be difficult to meet, flows should not fall below 810 cfs. Hopefully, at this level the fish that remain in the reach can wait out the period until more favorable conditions return with the end of the irrigation season. In winter, flows in the reach are generally not limiting. In fact, due to storage in upstream reservoirs, flows during recent years have been higher on average than they were historically. The recommendation is for a winter instream flow of 1,630 cfs to be maintained in all years except during periods of drought (lowest 20% of years) when the recommendation could be relaxed to 1,240 cfs. Spring Flows The earlier (1991) FWS report that dealt primarily with spring flow needs in the 15-mile reach described how the frequency of years with very high spring flows has greatly decreased since the early part of the century, and how years with low spring flows, once rare, are now commonplace. The magnitude of the annual peak day (average discharge over 24 hrs on the highest day of the year) was used to describe the attenuation of the spring hydrograph over time. To provide some idea of this change, the frequency of years with peaks in excess of a given amount can be compared between a block of years early in the century with a block of years later in the century. Years with peak runoff greater than 23,000 cfs occurred in 73% of the 41 years prior to 1943. Since 1954, years with a peak flow in excess of 23,000 cfs occurred only eight times, or 20% of the 40 years. For low-water years, a peak discharge less than 13,000 cfs never occurred in the 41 years prior to 1943, whereas since 1954, the annual peak flow was less than 13,000 cfs in 45% of the years (18 of 40 years). The year 1943 is used because many upstream water storage and trans-basin diversion projects began coming on line at that time. Though some of this change in the hydrograph may be attributable to changes in weather, the effect of water regulation has no doubt been significant. Using what is known about the life history of the Colorado squawfish and razorback sucker, FWS went on to discuss how this change in the magnitude of flows during the spring months could negatively affect the ability of these endangered fish to successfully reproduce and survive. Data were provided that demonstrated a relationship between the relative number of squawfish larvae produced in a year and the magnitude of the spring hydrograph: years of low spring runoff generally resulted in lower larval production. An explanation offered for this was that high flows are periodically needed to build cobble bars and flush fine sediment from the gravel/cobble substrates used by squawfish for spawning. Without sufficiently high flows, coarse particles become embedded in a tight matrix of silt and sand; the interstitial voids needed to protect deposited eggs are lost and egg-hatching success is reduced. Data also showed that razorback sucker spawning activity in the upper Colorado River is timed to coincide with the peak runoff period. Captures over the past 20 years indicate that most adults in spawning condition are found in warm, off-channel ponds and inundated floodplain habitats during the period of high water. Historically, these habitat types would have been extensive and available in most years. Today, potential sites are few and flows often do not reach levels high enough to inundate low-lying floodplain features. FWS also reported on observations made in the 15-mile reach during the drought years of 1988- 1990. During this period, backwaters, a low-velocity habitat important to both young and adult fish, were filling in with silt and sand because low spring flows were insufficient to flush fine xii