Problem : Dredging and the Lock and Dam System
The gravel found in the Lower Allegheny is the result of glacial deposition. This gravel, formed from igneous rocks, is hard, sharp, and clean, and therefore has commercial value (Watershed Atlas). The gravel in the Lower Allegheny also has a high silica content, which made it valuable in glass making. In the first decades of the 1900’s, 2.3 million tons of sand and gravel were being removed from the Lower Allegheny and upper Ohio Rivers.
Today, sand and gravel are still dredged for commercial purposes, as well as to maintain channels for commercial and recreational use. The Lower Allegheny has a system of nine locks and dams to allow navigation upstream. Historically used to transport lumber and commercial freight, these locks now mainly support recreational boating, particularly upstream. These structures disrupt the flow of water downstream, and have altered the habitat in the river. What was previously shallow riffles and pools has become a series of submerged riffles and deep pools, with aquatic life that resembles a lentic, rather than lotic ecosystem. Dredging and the commercial gravel trade also alter the ability of benthic organisms to find suitable substrate on which to live and feed.
The natural process of sediment transport downstream has effectively stopped, except in cases of extreme flooding. Once removed, the gravel is replaced with fine sand. Once this material settles on the bottom, it can trap organic material, which can create anoxic conditions in the stream. In addition, because smaller particles are more likely to be suspended in the water column, turbidity increases and affects the amount of light available for photosynthetic phytoplankton, algae, and vegetation. Increased sediment loads can also affect macroinvertebrate and fish populations. In the Lower Allegheny, Pool 5 has been dredged excessively, whereas Pool 6 was under a dredging moratorium throughout the 1990’s. When the fish populations in these two pools were analyzed, it was determined that fish productivity and survivability throughout the lifecycle was higher in Pool 6. (Pennsylvania Environmental Council 2001).
Recommendation 1
Dredging for commercial grave should be limited to one or two pools generated by the lock and dam system. Due to the fact that the different pools have different depths and different uses, some pools should not require excessive dredging to maintain their current user functions. While there will be continued loss of habitat in the dredged areas, limiting the disruption of natural habitat to a smaller portion of the watershed will increase the health of the watershed when considered as a whole.
Recommendation 2
The current use patterns of the upstream locks and dams should be analyzed. If the locks no longer need to support commercial freight, it may be possible to decrease the number of locks and dams, which would increase the flow in these areas. Removal of the locks and dams would be a primary step in restoring natural systems and flow in the Lower Allegheny.
References:
Pennsylvania Environmemtal Council (2001). Watershed Atlas. Retrieved March 9, 2011 from http://www.watershedatlas.org/lowerallegheny/fs_himp2.html
