You'll need more time for students own design for investigations. You may wish to consider using ideas from the River Cutters guide from GEMS at Lawrence Hall of Science.
Extensions Use the stream tables with holes blocked to investigate wave action and erosion.
The GEMS guide, Convection , is an excellent resource for enrichment activities. The USGS in Menlo Park sells topo maps to educators for half price if you bring a letter from your administrator on letterhead. The many pamphlets they have for teachers or students can be ordered in bulk from Colorado. Steve Carothers has an order form. There is also one at the Science Resources desk in the Curriculum Office.