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~, ~~ i~ January 1985 Riffle N C d E a m a .y L m C YI C N E 'v a N L d C b\ 137 POOI Size Fractions (mm) Fig. 6. Distributions of interstitial sediments (percentage of total weight) for representative riffle and pool tran- sects in the White River as compared to representative pool riffle transects in the Yampa River. The smallest two size fractions that are causing armoring in the White River sediments are 25%-30% of the total sediments. These fractions in the Yampa River are only 4% of the total. that larval fishes were captured only in the vicinity where squawfish were collected and that their movement corresponded to macro- invertebrate drift (2125-0100 hours). Five species of larval fishes emerged from this cobble site. Furthermore, inspection of the data collected at each seining site indicates a potential movement of fish from this same area at approximately the same time (Fig. 4), suggesting that the habitat was utilized by species other than Colorado squawfish. An important characteristic of this cobble area was the interstitial voids that contained little or no organics. The difference in drift above and below this area strongly suggests that larval fishes utilize these voids. Beames- derfer and Congleton (1982) observed the northern squawfish (Ptychocheilus orego- nensis) spawning in similar cobble and gravel sites in the St. Joe River, Idaho, where there was an absence of small substrates (sand) with adhering eggs up to 15 cm below the sub- LAMARRA ET AL.: SQUAWFISH strate surface. Prewitt et al. (1982) indicated that other similar habitats exist in the White River. However, their analysis of physical habitat and stream flow requirements for spawning squawfish considers only substrate size, not interstitial particles or voids. In the White River, the armored sediments (Fig. 6) prevent interstitial voids from developing, thus making the cobble habitat at Yampa River mile 16.5 unique in comparison to ob- served sites in other tributaries. It should be noted that Haynes and Muth (1982) did col- lect larval squawfish at Yampa River mile 17.8 on 14 August 1982. These fish were esti- mated to be 25 days old, indicating another spawning site above river mile 16.5. The physical factors that regulate the dis- tribution of organisms in stream environ- ments are varied. Such factors as nutrients (Hynes 1970), turbidity (Mann et al. 1972), temperature (Sprules 1947), light (Westlake 1966), and water velocity (McIntire 1966) w Q.V- Size Fractions (mm)