The valley begins in Elkridge Landing and runs north through a number of mill town communities until the river splits into two segments.

[2] As the river was not navigable beyond Elkridge, it was not a major path of commerce with only one ship listed as serving the northern branch, and four others operating around the mouth in 1723.

In 1952 an eight-foot wall of water swept the shops of Ellicott City and a 1956 flood inflicted heavy damage at the Bartigis Brothers plant.

[4] In 1972, Ellicott City and the Old Main Line railroad sustained serious damage as a result of rainfall from the remnants of Hurricane Agnes.

Many in the community cite the high amount of impervious surface and overdevelopment throughout greater Ellicott City to be a contributing issue to these types of floods.

This is where Francis Scott Key, while aboard a British ship, wrote "The Star-Spangled Banner," a poem later set to music as the national anthem of the United States.

[6] Conservation efforts have been made to protect this status through preservation easements along the Patapsco Valley State Park interface.