The Confederate Army established a defensive line after the Battle of First Manassas from the Occoquan to the Potomac River. They used the rivers to set up gun positions to stop Union boats. By December 1861 they had 37 heavy guns along the river.
On January 3rd 1862 the Union ships the USS Anacostia and USS Yankee shelled the Confederate location at Cockpit Point. Neither side made any headway.
Union ships would again approach the point on March 9th 1862, only to find the Confederates had left the area. But the Confederates had sealed off the Potomac River for almost five months.