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River was l30C at time of collection and the near- by Colorado River was B.50C. Numerous ripe male flannelmouth suckers were seined from the Paria River on April 21, 1975. A single ripe female was taken. Some ripe males were golden olive in color but others had a salmon colored band along the side and bright orange- salmon colored ventral fins and lower part of cau- dal fin. The Paria was 200C at the time of the collection and the nearby Colorado River was 90C. Two ripe males were collected on May I, 1976 from the Paria River. No ripe female was taken although some exhibited the bright orange-salmon coloration. A number of specimens had open wounds ranging from a dime to a half dollar in size. On May 1 the Paria varied from 250C to 130C at the mixing zone with the river. The Colorado was BOC. On May 2, 1976 another sample was taken from the Paria River but no ripe individuals of either sex was taken. However, one female had extensive bright coloration. The Paria River ranged from 240C to 220C in the collecting area and the Colo- rado River was BOC. On May 3, 1976 one small male and one large male flannelmouth sucker were found to be in ripe condition. No ripe female was collected although some exhibited breeding colors. Some individuals exhibited the non-nuptial silvery coloration. Some individuals had open sores simi- lar to the type described above. The Paria was 22.50C on May 3 and the Colorado was BOC. On May 3D, 1976 some of the many flannelmouth suckers were tuberculate. No ripe individual of eit~er sex was observed. Most individuals ap- peared to be spawned out. The Paria River ranged from 240C to 220C at the site of collection and the nearby Colorado River was 10oC. Tuberculate male flannelmouth suckers were col- lected on July 19, 1972 but none was ripe. The Paria was 24.50C and the Colorado was 120C. A collection of flannelmouth suckers was taken from the Paria River on August 7, 1975. Two fe- males and two males were cut open and the gonads were examined. The ova were of mixed size, small and granular. The testes were slender pinkish colored strands. Thus we interpreted both sexes to be in post spawning condition. A collection was obtained from the paria River on September 27, 1975. No tuberculate male was taken. On November 15 and 16, 1975 collecting was done in the Paria River but no flannelmouth sucker was taken. At 9:15 p.m. on November 15 the Paria was 60C and the main Colorado River 70C. At noon on November 16 the Paria was 60C and the main Colorado River was 10oC. The fluctuations in tem- perature of the Colorado are correlated with the releases from the Glen Canyon dam. We believe the flannelmouth suckers drop downstream after spawn- ing and many move out of the Paria into the main Colorado. When the temperature in the Paria drops below that of the Colorado then most if not all flannelmouth suckers move into the main Colorado. During several different collecting and observa- tion periods numerous individuals were observed moving upstream in the shallows above the deep pool at the mouth of the Paria. Although we did not investigate the upstream areas various hikers told us that large suckers were seen several miles up the paria. No specimen of the humpback sucker, xyrauchen texanus, was taken during the entire study. How- ever, we did collect three specimens of the hybrid combination Catostomus latipinnis x Xyrauchen tex- anus (Figure 1, lower). Our specimens agree with those described by Hubbs and Miller (1953). We believe the humpback sucker has been forced out of existence in the Grand Canyon section of the Colorado River. The low temperatures that con- tinue to prevail because of the base releases from the Glen Canyon dam are too cold for spawning. Secondly, the humpback sucker apparently does not utilize any of the small tributaries that are available in the Canyon area. Thirdly, the dam is a barrier that prevents upstream movement to suitable spawning areas. We postulate that during the early part of the filling of the Powell Reser- voir the dam acted as a barrier but water tempera- tures remained suitable for spawning thus hybridi- zation took place between the two forms. After the reservoir was filled and hydro-power operation be- FIGURE 1. (upper) Catostomus latipinnis TV 97489:372 rom SL. TU 94377:362 rom SL. 600 (lower) C. latipinnis X Xyrauchen texanus