March, 1963 Larimore & Smith: Fishes of Champaign County 



317 



by some standards, we have classed as 

 small rivers, figs. 2 and 3. They include 

 the lower Sangamon, the lower Salt Fork, 

 and the Middle Fork. They are perma- 

 nent streams that have less extreme and 

 less sudden fluctuations in water level 

 and temperature than creeks. Their flood- 

 plains and banks are generally shaded by 

 large trees, fig. 9. Like large creeks, the 

 small rivers contain both riffle and pool 

 habitats, table 4. 



Riffles of small rivers differ from those 

 of creeks in their greater volume of flow 

 and their greater proportion of large- 

 sized bottom materials. Because of the low 

 gradient of the small rivers, the riffles in 

 these streams are relatively fewer in num- 

 ber and occupy a smaller proportion of the 

 total water area than do those in creeks. 

 Sand and fine gravel riffles with little 

 aquatic vegetation occur in small rivers as 

 well as creeks ; because sand and fine 

 gravel occur also in many boulder and 

 rubble riffles that have some aquatic veg- 

 etation, the two types of riffles are less 

 clearly defined in rivers than in creeks. 



Pools in small rivers are either shallow 

 and have moderate water flow, or deep 

 and have sluggish current, fig. 9. Their 

 characteristics are similar to those of large 

 creeks, but the pools are deeper and have 

 more overhanging vegetation and greater 

 silt deposits. In certain parts of the lower 

 Salt Fork and lower Sangamon, occasional 

 pools are quite deep and have very slug- 

 gish water movements. 



Other Aquatic Habitats. — Other 

 Champaign County aquatic habitats that 

 do not fit into the above classification are 

 nonstream habitats, such as farm ponds, 

 artificial lakes, and oxbows. No natural 

 lakes and no permanent swamps and 

 marshes remain in the county. Ponds and 

 artificial lakes are of little interest in the 

 present study inasmuch as they have been 

 stocked, and they are of concern only if 

 the fishes they contain spill over into the 

 streams. Oxbows have characteristic la- 

 custrine populations. The component spe- 

 cies obviously were derived from the 

 streams with which the oxbows were once 

 associated. 



Changes in Stream Habitats 



The environment of a stream is sensi- 

 tive to almost any activity within the 



watershed. It is influenced by the human 

 population, agriculture, and industry as 

 outlined previously in this paper. 



For the 30-year period between the 

 studies of Forbes & Richardson and those 

 of Thompson & Hunt, specific measure- 

 ments or observations of the stream en- 

 vironment were limited to some studies 

 of stream-flow measurements and drain- 

 ing activities. From these studies and the 

 information on the general development 

 of the county, we know that many water 

 habitats were actually eliminated and that 

 draining and dredging resulted in in- 

 creased fluctuation of water levels, in- 

 creased turbidity, and a reduction in aquat- 

 ic vegetation. We know that channel 

 straightening, with the elimination of 

 meanders, actually shortened stream 

 courses in many areas and consequently 

 increased the stream gradient. The re- 

 placement of stagnant-water marshes by 

 underground drains that discharge waters 

 that are relatively cool in summer and 

 warm in winter may have reduced seasonal 

 fluctuations of stream temperatures. 



Between the investigations of Thomp- 

 son & Hunt in 1928 and the present time, 

 we have specific information on certain 

 changes that have occurred. Thompson & 

 Hunt's original field notes provide an un- 

 usual opportunity for evaluating various 

 changes in habitats at identical, or nearly 

 identical, collecting sites, figs. 10 and 11. 

 Habitat differences can be seen at specific 

 sites described by Thompson & Hunt and 

 then examined 30 years later during the 

 1959 investigation. These differences have 

 been evaluated and summarized in table 5. 



The principal changes noted have to do 

 with dimensions, particularly in average 

 depth and average width of the pools 

 where collections were made. The field 

 work of the 1928 survey was carried 

 on "from early spring to late autumn" 

 (Thompson & Hunt 1930:14) ; most of 

 our collections were concentrated in the 

 dry months of late summer. The entire 

 summer of 1959 was considerably drier 

 than that of 1928. Despite differences in 

 the time of field work and in the amount 

 of precipitation in 1928 and 1959, the 

 two censuses disclosed that measurable 

 changes had taken place in the Champaign 

 County drainage systems in the years be- 

 tween the censuses, table 5. 



