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During the field work at Mathers Hole, a radio-tagged humpback chub occupied the pool between cross sections 9 and 10 for three days and exhibited a high degree of fidelity to the location even after being disturbed by surveying activities (J. Hawkins, pars. comm). A razorback sucker was observed porpoising in shallow water at the downstream end of the pool between cross sections 12.1 and 14. 3.1.1. Bar Morphology The bar at Mathers Hole is formed and maintained by backwater conditions during high discharges when the threshold for transport of the constituent gravels and cobbles is exceeded in non- backwatered reaches upstream. Primary evidence for the backwater control is the fact that the bar is bank attached on the outside of the bend. The location and characteristics of the bar are controlled by the geometry of the bedrock outcrop (Weber Sandstone) primarily on the left side of the valley and a relatively low elevation strath terrace on the right bank of the low flow channel from cross section (XS) 10 to XS 5. High-water marks from the 1984 peak discharge indicate that the flows did not overtop the strath terrace, but it is apparent based on the elevations of the s1ackwater deposits that floods have in the past overtopped the strath terrace (Elevation 105 ft). In situ bedrock was located in the bed of the low water channel at XS 11 which tends to suggest that the thickness of the alluvium in the reach is probably not great. The Weber Sandstone dips to the south-southeast and the alluvium probably only forms a fairly thin veneer over the outcrop. The bar at Mathers Hole is flanked by two chute channels. The right chute channel is the low flow channel and the left chute channel only flows at higher discharges. The differences in thalweg elevation between the two chute channels range from 9 to 10 feet at XS 6 and XS 7, respectively to 6 feet at XS 10 and 1 ft at XS 11. The chute channels converge together at the downstream end of the bar at about XS 5. A detailed geomorphic map of the study reach was constructed in the field (FIgure 3.2). At the upstream end of the reach at cross section 14 the valley width is constricted by large colluvial boulders (> 5m) that originated from a mass faUure of the Weber Sandstone that forms the valley wall. A Weber Sandstone cliff that is somewhat overhanging and rises about 50 feet above the thalweg forms the left bank of the channel and the valley wall. The valley wall and the left bank of the channel are coincident from XS 14 to XS 12.1. In contrast, the right bank of the channel between XS 14 and XS 10 is comprised of alluvium, and the bankfull capacity of the channel in this reach is about 18,000 ets. The sand dominated vegetated alluvial surface was not overtopped in 1993, but 1984 flood debris is present on the surface. Aows that overtop the alluvium on the right bank are confined by the wiley wall that changes from the bedrock cliff at XS 14 to a strath terrace at XS 10. The strath terrace tread is about 9 to 10 feet above the alluvial surface and from XS 10 to XS 5 strath terrace forms the right bank of the 3.3 Resource Consultants & Engineers, Inc.