239 JTUs at Huntley, Miles City, and Sidney, respectively (U.S. 

 Geological Survey 1975) . Turbidity increases in the Yellowstone 

 River downstream from the Powder River. Turbidity in the lower 

 Powder River averaged 714 JTUs for 7 samples taken from March 

 through September in 197 5. 



The Yellowstone River supports a trout fishery in the upper 

 reach and a warmwater fishery in the lower reach. Diversity of 

 species increases progressively downstream. Peterman and Haddix 

 (1975) reported that eleven fish species (5 families) have been 

 recorded in the upper Yellowstone River in Montana, 20 species (8 

 families) in the middle river, and 46 species (12 families) in the 

 lower river. Two additional species were collected from the 

 Yellowstone River in 1980; rainbow smelt f Osmerus mordax) and white 

 bass (Morone chrysops) (Table 1.) 



Newell (1976) determined that a rich aquatic invertebrate 

 population is present in the Yellowstone River with both number of 

 species and standing crop decreasing from the upper to the lower 

 river. Mayflies ( Ephemeroptera ) , caddisflies ( Trichoptera ) , and 

 true flies ( Diptera ) dominated the bottom fauna. The stonefly 

 fauna ( Plecoptera ) was diverse but not abundant and decreased in 

 number of species downstream. 



The present study encompassed the lower half of the 

 Yellowstone River from Huntley, Montana (river km 566) downstream 

 to the North Dakota border (approximately river km 18) (Figure 1) . 

 Major tributaries along the lower river are the Big Horn River 

 (river km 476) , Tongue River (river km 298) , and Powder River 



