A dam behind Dishaw's foundry on the Grasse River could leave the river too low to spill over during a prolonged dry spell, while water still ran through a raceway and formed a pool in a rocky basin below, where people swam and crossed the raceway on a plank bridge. Around Falls Island, the river's three dams were crib dams built from heavy timbers laid like a log house and filled with stone as an economical though not permanent way to hold back water for power. Their construction began with holes drilled into bedrock, where split iron rods or bolts with wedges were driven in, foundation logs were dropped over the bolts and anchored, and successive courses of large logs were laid at right angles, spiked together with iron drift bolts, and raised to four to six feet high before the spaces were filled with stone and gravel. The dam face was vertical, sloped upstream, and was finished with planking so ice and debris could pass over. When river levels are low, some of the anchor bolts remain visible, bent over by years of ice and logs. Early in the 20th century, industrial activity on the island was ending, and with the gristmill no longer operating, the sash and blind building torn down, and the lumber mill burned out, the dams soon fell into disrepair. Although there was discussion in the 1930s of putting the dams back into operation, nothing came of it.