Henry Shreve’s Great Dam Removal Experiment of 1831
Removal of obsolete dams to restore watersheds to their natural working conditions is a major topic these days in environmental news. Many of us think of such dams as local perturbations on a stream that act as sediment traps and impediments to the natural migration of aquatic life. Fine-grained silt particles in the sediment collecting behind the dam retain a potentially dangerous reservoir for fertilizer and pesticide chemicals washing in from fields and residences. But the situation is much more complicated than that because the function of streams as sediment transport systems depends on a delicate equilibrium that extends over the entire length of the watershed. You can see how this works by recognizing that the force moving sediment is given by the slope of the streambed. If a location has more force available than needed, the extra force will allow the water to eat away at the banks to create meander bends. This, in turn, effectively lengthens the channel to reduce the slope. If there is not enough slope to move the sediment, then gravel-bar deposits will build up producing a braided streambed with an increased slope and an increased sediment load capacity. Of course, the process is a [...]