108 



to the bottom-lands, of which about 17 square miles have been 

 cleared of the forests and placed under cultivation, 7 represent 

 river and lakes at low-water stage, 10 are permanent marshes, 

 and the remaining 22.5 are covered by forests, and lie at so low 

 a level as to be subject to frequent overflow. It is difficult to 

 distinguish between marsh and woodland in those areas 

 covered by low growths of willow (Salix nigra) or sparsely 

 wooded with willow-trees of considerable size. As their low 

 elevation renders them subject to overflow on slight rises of 

 the river, and as the vegetation is usually of a semiaquatic 

 nature they are on our map in large part included in areas 

 designated as marsh. As the elevation increases, maples 

 (Acer dasycarpwn) appear among the willows, then the green 

 ashes (Fraxinus viridis), while in the higher bottoms the elms 

 ( Ulmus americana) form the major part of the forest, with 

 maples, box-elders (Negundo aceroides) , sycamore (Platanus occi- 

 dentalis),\)eca,ns(Carya olivceformis), oaks (Quercus palustris), and 

 clusters of cottonwoods (Populus monilifera) interspersed. The 

 marshes and lagoons, especially along the lower part of the 

 river, are often fringed with dense and impassable thickets 

 of button-bush (Cephalanthus occidentalis) and buckbrush (For- 

 esteria acuminata). 



Cooley ('91) gives the average height of the banks of the 

 Illinois above low-water level as 10 feet for the region between 

 Copperas Creek and Havana and 11 feet from Havana to the 

 mouth of the Sangamon River. The range in height for the 

 same distance is 7 to 12 and 9 to 12 feet respectively. The 

 greater height below Havana is doubtless due to the deposits 

 contributed by Spoon River. The dam at La Grange, com- 

 pleted October 10, 1889, has raised the water about two feet at 

 Havana, so that the actual height of the banks above the water 

 at its lowest stage, under present conditions, averages less than 

 8 feet. The immediate banks are usually higher than the ad- 

 jacent bottom-lands. This is true of the tributary streams also, 

 and is especially well marked in the case of Spoon River, 

 which carries large amounts of sediment at times of flood. It 



