(dave morlan, emergency management coordinator for boone county) "as that ice flow came down, it and hit the bridge. it actually tilted a couple of pile-ons." ice jams form in bends and shallow areas of rivers. large pieces of ice stick together on the river...creating a dam of ice. and like dams...ice jams keep water back. (dave morlan, emergency management coordinator for boone county) "ice jams are hard to predict where they're going to be and if they're going to happen." and once they break free...like this weekend in central iowa with warmer temps...they come crashing downstream...with lots of water and ice. (dave morlan, emergency management coordinator for boone county) "you can have several miles of ice backed up that causes a lot of flooding. they're very dangerous -- got a lot of power to them." further downstream at ledges state park...look how much ice has accumulated on the river. (klint peterson, ledges park ranger) "due to all the warm weather and the ice and snow melting, it's all funneling down causing the flood at lower ledges." forcing park rangers closed canyon dr