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.ft- 44 GREAT BASIN NATURALIST MEMOIRS 197 Verkin Springs, entering just below the Hur- ricane Diversion, plus inflow from La Verkin and Ash Creeks, maintain permanent stream flow downstream to Washington Diversion (Fig. 1). Littlefield Springs, entering at the lower end of the narrows, maintain per- manent streamflow downstream to the Mes- quite Diversion (Fig. 1). When the total streamflow is actually used at the above di- version points, only about 52.5 km (or 29 per- cent) of the remaining 180 km of potential habitat for the two endangered species re- stricted to the mainstream is actually con- stantly available to them. The narrows (Fig. 1) divides the main- stream into an upper and a lower component that appears to effectively isolate the con- tai.ned fish populations. Elevation and cli- mate in the two regions differ significantly. The difference was reflected by the nearly one-month earlier spawning of the woundfin population in the lower river in the spring of 1977 (Fig. 7). The question of requirements of these fish- es in their remaining habitats has been the subject of studies conducted at various levels 30 25 - 23Aug. 1977 (N=70) 0--.0 25-26 Nov. 1977 (N=370) 20 z o ~ S 15 :::> ll.. o ll.. 't5 10 ~ o 18 20 30 40 50 No.3 of intensity since 1961 (Cross 1975, 78, Wil- liams 1977, Schumann 1978, Peters 1970, Lockhart 1979, Vaughn Hansen Associates 1977). The drought of 1977 resulted in some of the lowest flows on record in the Virgin River, a circumstance which allowed signifi- cant insights into the probable effects of wa- ter development projects which would tend to reduce or alter flows in the river. The more normal flows of 1978 provided a useful comparison to the low-flow conditions of 1977. Length-frequency analysis was used as a convenient means of examining the popu- lation structure of the fishes in the Virgin River. Samples were taken by repetitively seining an area until the number of fish col- lected amounted to less than 10 percent of the highest number collected. In this way we insured a good representative sample of all fish occurring in the sampled area. Figure 2 demonstrates that samples taken in August 1977 and more extensive sampling from No- vember 1977 provide essentially the same picture of population structure for woundfin. This suggests that sampling done in both Au- gust prm Strul veal prisc Prof the pOpl narr, that latio 'W stunl fact( an e sumI for j WOUl lengl SUCCt pres( wow , ~ Z 0 2 ~ <r ....J ::> Q.. 0 Q.. - 0 ~ 0 60 70 80 90 SI ZE Fig. 2. Length frequency of woundfin in Virgin River above the narrows during fall 1977. Fig.