criteria developed in 1978 and 1979 than 1977 (Figure 27) . 

 However, the relative amount of top width available for spawning at 

 various flows was very similar for both curves. At a discharge of 

 less than 140 m /s (5,000 cfs) the amount of top width available 

 declined sharply. Optimum flows appeared to be between 170 and 310 

 m /sec (6,000 and 11,000 cfs). Amount of top width suitable for 

 spawning continually declined at flows greater than 310 m /s 

 (11,000 cfs). The historical median flow for the Yellowstone 

 during April is 9,170 cfs (from flow duration hydrograph compiled 

 by the U.S. Geological Survey). 



Tongue and Powder Rivers 

 Location, Substrate, and Depth 



The substrate of the Tongue and Powder rivers was dominated by 

 fine pebbles often intermixed with particles of coal of similar 

 size. There were also a few runs with boulders and cobble 

 dominating the substrate. Inside bends and slack water areas were 

 composed of sand. 



Although the exact location of sauger reproduction in the 

 Tongue and Powder rivers was not pin pointed by the collection of 

 eggs, large numbers of ripe males were regularly collected in runs, 

 and riffle areas less than 1.2 meters in depth. Substrate on these 

 areas was fine pebble. During daylight electrof ishing, females 

 were regularly collected in pools, almost all of which were not 

 deeper than 1.2 to 1.5 m. Nelson (1968a) found sauger commonly 

 spawning in water less than 0.6 m and collected eggs 0.6 to 3.7 m 

 below the maximum water surface elevation. Rawson and Scholl 



90 



