A section of the Mississippi River between miles 719 and 722 upstream from St. Louis was used to study large river ecosystems. The flow of the river during the time of the study was between 40,000 and 70,000 cfs. The dimensions of the area and the paths of travel along which the movement of dye was studied are shown. Water along these paths does not intermix. One path is a continuation from upstream of the main navigation channel of the river. Tracer studies indicate that the water moving along this path through the study section has little interaction with the immediate shore zone. The limnological character of this water has been largely fixed by upstream mixing prior to entry on the path. Another path drains an inshore area that has a rather regular beach boundary upstream. Along this course, there is an input of water from shallow areas. These shallow backwaters are of particular limnological interest since the production upon which the food-web of the ecosystem is built appears to occur here. The areas of main current are likely avenues for the dispersal of this production.