p 



Fig. 6. — Finger- 

 nail clams, important 

 food items for some 

 fish and birds, have 

 virtually disappeared 

 from the river above 

 Beardstown. 



ppm. In 1913 and 1915 Richardson (1925:381) col- 

 lected an occasional Hcxagcnia nymph in middle and 

 lower Peoria Lake; after 1915 he did not find any in 

 the river above Havana. 



Mayfly emergences can be spectacular when millions 

 of these insects are drawn to lights and may concentrate 

 in such numbers as to be hazardous to traffic. Such 

 emergences have been characteristic of many parts of 

 the Illinois River. The last time we observed a large 

 emergence at Havana was in 1949. Paloumpis & Starrett 

 (1960:419) collected Hexagenia limbata nymphs in the 

 1950's at Quiver Lake, and in most summers during the 

 early 1950's they saw subimagos at or in the immediate 

 vicinity of the lake. Since the late 1950's we have not 

 observed any mayfly emergences at the lake and Star- 

 rett & Paloumpis (unpublished) did not collect any 

 nymphs there in 1964. They collected some Hcxagcnia, 

 though, in 1964, in the river below Beardstown and in 

 the Alton Pool. 



The predominant organisms in the 1964-1965 bcn- 

 ihic sam]3les were jJoUution worms and these were quite 

 abundant, even in the samples from below Beardstown 

 where both mayfly nymphs and fingernail clams were 

 taken (Starrett & Paloumpis, unpublished). In 1915 at 

 Lake Matanzas, below Havana. Richardson ( 1921 a: 506- 

 507) collected only 4.4 worms \kv sc|uare yard, whereas 

 Paloumpis & Starrett (1960:430) in 1953 look 11.007 

 per scjuare yard in the same area. 



Such drastic changes in the bcnlhic populations as 

 those described above can be accounted for only by the 

 accunuilative effect of pollution in the bottom muds of 

 tlie Illinois River waters. Such changes may not l)c 

 shown by chemical analyses. 



AQUATIC VEGETATION 



Within the last 15 years there have been unusual 

 changes in the aquatic vegetation of the Illinois River 

 and its bottomland lakes. Today there is a vastly dif- 

 ferent picture in the vicinity of Havana from thai 



painted by Kofoid at the turn of the century. Flag and 

 Thompson lakes disappeared in the early 1920's, the 

 result of the building of levees, drainage, and cultivation. 



Kofoid (19():i:236) slated: '"The ac|uatic environ- 

 ment at Havana im|}resses the \isiting biologist who for 

 the first time traverses its river, lakes, and marshes, as 

 one of exceedingly abundant vegelalion. indeed almost 

 tropic in its luxuriance. . . . He will find acres upon 

 acres of 'moss,' as the fishermen call it — a dense mat of 

 mingled Ccratophyllum and Elodca choking many of 

 the lakes from shore to shore, and rendering travel by 

 boat a tedious and laborious process. . . . The carpels 

 of Lcmnaccac will be surprising, and the gigantic growths 

 of the semiaquatic Polygonums will furnish evidence of 

 the fertility of their environment." 



The first big change in aquatic vegetation came 

 shortly after Kofoid had completed his study, with the 

 1900 diversion of Lake Michigan waters into the ri\er. 



Richardson (1921b:46) recorded the disappearance 

 of aquatic plants from Peoria Lake in 1920 as follows: 

 "The luxuriant growths of coarse aquatic plants ( Pola- 

 mogeton, Ceratophyllum, Scirpus, Vallisineria, etc.) that 

 covered several .square miles of Peoria Lake at mid- 

 summer and autumn levels between 1910 and 1914, 

 and their rich fauna of small invertebrates along \\ith 

 them have disappeared now altogether in the upper and 

 middle lake except for an occasional scraggly clump at 

 the very edge. In the lower lake, a thin patch of 

 Potamogeton and Ccratophyllum, covering less than two 

 acres, was still growing in a small springy slough, . . ." 



Thompson (1928:304) reported that pondweeds 

 (Potamogctons) and other large aquatic plants in Peoria 

 Lake, the river, and connecting sloughs and lakes down- 

 stream, disappeared almost completely between 1915 

 and 1920. But about 1922 pondweeds began to reap- 

 pear and increase rather rajiidly in many areas of Peoria 

 Lake. 



Purdy (1930:113) slated that in 1921 Peoria Lake 

 held some growths of pondweeds and algae, but these 

 were so slight as to be overlooked by the casual obseiA'cr. 



From the late 1930"s to the middle 1950's, in some 

 places along the central stretches of river there was an 

 abiuulance of aquatic vegetation, but this has now al- 

 most completely disappeared. The reasons for this are 

 not clearly understood, Thcic may be some inimical 

 materials in the waters now, and it appears that the silta- 

 tion of the last decade has been a factor. Siltation affects 

 aiiuatic jjlants adversely in two ways: it produces a 

 tiubidity which reduces the penetration of light and in- 

 hibits photosynthesis, and it creates bottom conditions 

 which make it difficult or ini]Jossible for various species 

 of jilaiits to obtain anchoiage whc-n they are buffeted by 

 wave action. 



The importance of wave and fish action in rc- 

 suspending sediment particles in Lake Chautau(|ua has 

 been jjointed out iiy Jackson & Starrett (1959). Dining 

 the spring of 1953 tliey found that, with an increase in 



13 



