Downgraded Tropical Storm Isaias started to impact Southern Maryland at 1:00 AM on the 4th of August 2020, first by light rain before the real outer bands started to cause flooding and tornado damage over the progression of 7 hours.  


Southern Maryland experienced 3 confirmed tornadoes:

EF1 100mph wind speeds 7.3-mile track - Piney Point to Leonardtown

EF1 90mph wind speeds 2.2-mile track - Dares Beach to Willows

EF0 80mph wind speeds 0.9-mile track - Point Lookout to Piney Point

Calvert County Maryland also experienced 1 tornado, the Eastern Shore had 4 confirmed tornadoes, 3 in Maryland and 1 in Delaware. With the number of tornadoes in Maryland, the event has been named the August 4th Tornado Outbreak.  

Charles County Route 6 bridge over Persimmon Creek and St. Mary's County route 5 located on Saint Clement creek, partially collapsed due to erosion triggered by flash flooding. 


Tropical Storm Isaias triggered moderate to severe flooding events through Southern Maryland. The soil was already high in moisture as a strong line of storms crossed the basin several hours before at 3:20 pm. Isaias reached Southern Maryland at around 1:00 am, in the beginning, light rain was falling but the steady increase added to the unavoidable flooding risk. 


All the rainwater rapidly entered the unconfined aquifers below St. Mary's County, the water table was then forced to rise along with it.  

The below images represent surface water flowing downhill through a small lake and out into Breton Bay through a stormwater drainage pipe, this took lots of sediment out with it. This repeated along other properties facing the bay until the entirety of Breton Bay was the orange/brown colour of the eroding Lowland and Inland Deposits.  


Breton Bay lies within Pleistocene/Holocene (quaternary) Lowland Deposits consisting of Gravel, Sand, Silt, and clay. The base layers are medium/coarse-grained sand and gravel, along with cobbles and boulders. The whole 'formation' is 0-150 feet (0-45 meters) in thickness and is known to form cliffs along waterways. 


This 'formation' commonly contains reworked Eocene (56-33 million-year-old) glauconite which is an iron/potassium phyllosilicate associated with mica-based minerals, it is usually green in colour and has a very low weathering resistance hence why it's reworked.


Lignite Silty Clay and Siltstone come in various colours, the clay is most commonly visible in orange/rust and dark grey to grey colouring.


Clay in the cliffs of Breton Bay sometimes resembles lignite which is the lowest form of coal sometimes referred to as brown coal, its peat (partially carbonized plant mass) which has turned to rock and has a carbon content of 60-70%. 

Sometimes fossils deposited in an estuary (where a river meets the ocean) or marine setting are uncovered through the erosion of cliff structures along the bay. 


Images (Left): siltstone, (Middle, Right): Clay


With the amount of rain falling some of the clay/siltstone cliffs lining Breton Bay eventually failed, these sediments are very young and make up the mostly Holocene section of the Atlantic Coastal Plain province in Breton Bay. 


Since a lot of Breton Bay is comprised of clay, the added floodwaters tuned the oversaturated sediment into a slurry as clay spreads evenly through water. Cliff faces collapsed as the clay slid out of place and down towards the water's edge, this undercut parts of the above topsoil which in turn broke loose and fell towards the bay. 



One location in Breton Bay experienced ground subsidence in the form of a sinkhole as one opened up close to the cliffs along the bay. 

The images below show a Cover-Collapse Sinkhole, a relatively common type. It is possible that this one developed over the progression of several hours after or during Tropical Storm Isaias as the water started to drain through the permeable layers below. 

The sandy clay cover sequence below the topsoil made an ideal environment for a sinkhole to form. Sediments at the base would have spalled into a cavity over time than created a natural arch, the cavity continued to migrate upwards until it eventually breached the surface and the ground above finally gave way.   


Groundwater moves in certain patterns underground and after a major rain event, there is a lot of it. With some layers of soil and rock beneath the surface being more porous than others rainwater can move through it. The movement is not completely straight down but horizontally as well since the water moves through permeable layers.

In Breton Bay, this is visible as very small streams were conjoining in the talus slope created by the landslides. patterns in the beach sand also show signs of water flow from the porous rock layers exposed by Tropical Storm Isaias.  



Water Management Administration Water Supply Program December 2005, Source water Assessment for St. Mary's County

Maryland Geologic Survey. St. Mary's County 1968. Geologic maps of Maryland.

Dénes Lóczy, Szabolcs Czigány and Ervin Pirkhoffer. Flash Flood HazardsInstitute of Environmental Sciences, University of Pécs


Hartner, Z. (2020, August 8). Tropical Storm Isaias leaves some Maryland roads in tatters. WTOP.

Groundwater Flows Underground. (n.d.). USGS. 

Sinkholes. (n.d.). USGS. 

Daniel J. Soeder, Jeff P. Raffensperger, and Mark R. Nardi.(2007). Effects of Withdrawals on Ground-Water Levels in Southern Maryland and the Adjacent Eastern Shore, 1980–2005. U.S. Department of the Interior, U.S. Geological Survey, Reston, Virginia.

Thomas C. Winter Judson W. Harvey O. Lehn Franke William M. Alley. Ground Water and Surface Water A Single Resource. U.S. Geological Survey Circular 1139