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128 GREAT BASIN NATURALIST just prior to seining. Drift material was pre- served in 10°Io formalin. Physical data at each seining sample loca- tion (water temperature, water depth, cur- rent velocity, and dominant substrate type) were also determined. A sediment core sample was also collected at each seining sample location using a 4.6-cm-diameter cor- ing device. These interstitial sediment sam- ples were transported to the laboratory and analyzed for size fractions and organic content. Three cross-river transacts at river mile 16.5 and two at river mile 18.0 were estab- lished. Water depth, current velocity (0.6 dis- tance from the bottom), and dominant sub- strate were recorded at transect points every 2 m across the stream. At five equidistant points on each transect, benthic macroin- vertebrates and chlorophyll a (periphyton) samples were collected. In areas deeper than 1 m, SCUBA was used to sample. All chloro- phyll samples were stored on dry ice for transportation. Laboratory In the laboratory fish stomachs were re- moved, and the contents were placed in vials containing 70P/o ethanol. In fishes with dis- tinct stomachs, such as Ictalurids and some Cyprinids, only the contents of the stomach Vol. 45, No. 1 were taken. In fishes with poorly defined stomachs, such as Catostomids, the anterior portion of the gut from the esophagus to the first bend or distinct constriction of intestine was taken for analysis. Total displaced vol- umes of stomach contents were measured with a graduated volumetric tube to the nearest 0.01 ml. Contents were then identi- fied, counted, and measured to the nearest millimeter. Drift net and benthic samples were floated in a sugar-water solution and poured through a 0.25 mm sieve to separate the organic and inorganic portions. Insects and fishes were then manually sorted from all other organic materials. These items were preserved in 70% ethanol, identified, and counted. The remain- ing debris was dried at 105 C for 24 hours and weighed on a Sartorius balance. All ben- thic insects were additionally measured to the nearest millimeter. Interstitial substrate samples collected in the field were dried at 105 C for 24 hours. The sediments were then sieved into five size fractions (see footnote in Table 1), which were each weighed on a Sartorius toploading balance. The 0.25-mm size fraction was re- dried for 24 hours at 105 C, cooled in a des- iccator, and weighed to the nearest 0.01 mg on an analytical balance. These samples were then asked in a preheated muffle furnace at 550 C for 20 minutes, cooled, and reweighed. TABLE 1. Physical and biological data summary for five transacts on the Yampa River between river miles 16.5 and 18.0. Figures are an average of river cross-sections with standard deviation. Sediment Transact mean 1 SD mean 2 SD mean 3a SD mean 3b SD -mean 4 SD mean 6 SD Dominant as a percent Chl a Depth Velocity substrate (mg/m~) (m) (m/sec) (em) A B 12.2 0.9 0.63 9.0 92.6 6.0 11.9 0.2 0.2 3.4 35.5 7.9 31.6 1.4 0.3 0.3 -0- 6.0 15.1 0.9 02 0.4 -0- 14.7 9.6 0.3 2.4 7.5 67.0 20.0 -- 0.1 1.0 -0- 11.3 42 30.0 0.4 2.1 5.8 70.0 18.0 2.3 0.1 0.7 2.9 10.8 5.6 7.8 2.3 0.6 6.3 27.2 0.2 13.1 1.7 0.6 7.9 43.2 0.5 8.8 0.3 3.5 9.0 74.2 13.4 4.6 0,2 1.6 3.4 19.5 9.7 .s ,t °Sediment size Fractions (mm): A = > 14.7 B= 19.7-4,0 C = 4.0 - 0.5 D = 0.5 - 0.25 E ~ <0.25