Physical monitoring and sediment mapping survey of the Patapsco River near Bloede Dam, Howard and Baltimore Counties, Maryland
2017, Van Ryswick, S., Sylvia, E.R., Knippler, K.A., Gillmor, A., and Connallon, C.
Coastal and Estuarine Geology File Report No. 16-07
Bloede Dam is located on the Patapsco River between Baltimore County and Howard County, Maryland. The Dam was constructed in 1906-1907 for the purpose of hydroelectric power generation by Consolidated Gas Electric Light and Power Company (Ortt and Sylvia, 2012). Shortly after its construction, sediments began to accumulate behind the Dam prompting a routine operation of maintenance and dredging (Ortt and Sylvia, 2012). In 1914, the sediment depth behind the dam was documented at 12.5 feet (Ortt and Sylvia, 2012). In 1924, the hydroelectric generation operation ceased, and presumably all sediment maintenance behind the dam also ceased at that time (Ortt and Sylvia, 2012). Currently, the Maryland Department of Natural Resources (DNR) owns the Dam. DNR, along with American Rivers and the National Oceanic and Atmospheric Administration (NOAA), are collecting information to determine possible alternative uses of the Dam including the potential removal of the Dam.
Physical and Geological Setting
The Patapsco River Valley between Ellicott City and Bloede Dam is underlain by ancient, very hard rock formations. The major rock type around Ellicott City is the Ellicott City Granite, which is about 420 million years old. It is used as a building stone and is seen extensively in old Ellicott City construction. South of Ellicott City, the river has eroded through the valley into the much older Baltimore Gabbro Complex, which is about 750 million years old. This dark, dense rock is also valued for construction. Throughout the valley are numerous mineral veins and deposits within the bedrock. Quarry and mining operations in the Patapsco Valley were common in the 18th and 19th century, and were an important part of the regional economy.
Soils within the river valley are derived from the bedrock erosion. Most of the rock erodes to sandy and gravelly sediments, with relatively little clayey material remaining. While the bedrock produces clay and iron-rich fine sediments, most of these are swept away by water. The valley walls are steep enough in many areas to prevent thick accumulations of fine-grained soils.
The objectives for this study included:
- Survey 30 monumented cross-sections across the Patapsco River and complete facies maps by utilizing pebble counts and analyzing grab samples in the laboratory;
- Produce a digital elevation model (DEM) of the Bloede Dam impoundment; and
- Take photographs to document visual topographic and sedimentological changes in and around the river along each of the transect lines.