In rivers and streams with any current there may be little unconsolidated sediment that can be vibracored. What sediment there is should be located along the stream margins, on the inside of bends, and in quiet water behind obstructions. Before attempting to core, it's often helpful to probe the bottom with a length of half-inch steel pipe or conduit. As a general rule of thumb, the length of narrow pipe that can be pushed into the bottom is some indication of the vibrocore length to be expected. In firmer sand, gravel or clay substrates, a shorter core tube works better than a longer one, assuming the core tube meets refusal either way.