DESCRIPTION OF THE AREA 
Except for pools 1, 2, and 3 and the recently constructed 
"Chain-of-Rocks" Dam near St. Louis, Mo., all of the locks and 
dams constructed by the Corps of Engineers in connection with 
the 9-foot navigation channel project on the Upper Mississippi 
River are located in the area under study. Thirteen of the dams 
are on the Upper Mississippi Refuge, and 10 are on Mark Twain 
Refuge. 
At the time the Upper Mississippi Refuge was established, 
the river bottoms included a large number of wooded islands, 
with deep, intervening sloughs the rule. Hundreds of lakes and 
ponds were scattered through the wooded areas. There were some 
hay meadows on the islands, together with small farming operations, 
put the bottoms were essentially wooded. The marshes were confined 
to the shores of lakes and guts leading off the sloughs. Marsh 
flora was also limited and the marshes tended to dry up before 
the end of summer. Many lakes and ponds annually dried up con- 
pletely, while water levels in others receded markedly. 
Constant drying out of marshes and ponds resulted in con- 
siderable loss of marsh and aquatic species, especially the 
annual plants. Consequently, waterfowl habitat was quite limited. 
In the early 1930's, the Corps of Engineers initiated the 9- 
foot channel project, which ultimately resulted in the present 
series of 26 locks and dams. These impoundments abruptly changed 
the river bottoms from an area of wide fluctuation in water 
levels to an area of semistabilized water. While spring floods 
still occur, the bottoms do not dry out in the sumer. Lack of 
marsh and aquatic plants is no longer a problem, especially north 
of the Wisconsin River. Old hay meadows and timbered areas are 
now permanent marshes providing excellent habitat for waterfowl. 
There is considerable variation in habitat over the 561 miles 
of the Upper Mississippi and Mark Twain Refuges. The best marsh 
habitat is found north of the mouth of the Wisconsin River on the 
Upper Mississippi Refuge. Below the Wisconsin River, few extensive 
marshes have developed to date. 
Above the mouth of the Wisconsin River, physical features 
necessitated construction of a series of comparatively short navi- 
gation pools. Seven pools are located in the upper 50 miles of 
the Refuge. In these short pools the ecological picture has 
changed to a marked extent. From pool 7 south, much longer pools 
(averaging 30 or more miles in length) are found. Here the 
natural ecology has not undergone such a marked change. 
