These erratics in the moraine were once buried deep in the till fabric. As the river flowed over the moraine it slowly eroded away the till fabric. However, since this is a relatively small river only the smaller material present within the till has been eroded away. There for the material left over in the bed of the river are the large glacial erratics that would take huge velocities of water to move. Since it is very unusual to get high enough velocities to move erratics the generally stay within the river bed. There they slow the velocity of the river and cause material to be deposited behind the boulder. Here are more erratics, although the image does not let you see the deposition that occurs behind these boulders, believe me it is there.
There are certain times during the year when more erosion is done and other times when deposition is more prevalent. During the spring when the winter's snow is melting the rivers depth, discharge, and velocity increase, this is a period of much erosion. During the summer the discharge of the river can fluctuate greatly. Times of drought and little rainfall cause the river to have a low discharge and small velocity which creates a period of overall deposition within the stream bed. Large rain storms can increase the rivers discharge by large amounts which in turn can do much erosion within the stream bed. Here is an example of bank high flow during the summer time.
Along with this large surface rainfall and increase in discharge, comes a large suspended load within the rivers. The suspended load can do much erosion by colliding with materials and breaking them free from there original resting place. Suspended load can be measured through much field work, using precise instruments and is topic of much research done here at the University of Wisconsin at Stevens Point. Here is an image of suspended load being measured during a high discharge period in Steadman Park.
A large part of this data and information was donated by Dr. Karen Lemke of the Geography/Geology department at the University of Wisconsin at Stevens Point. We thank her for her help.