I was listening to a podcast discussing Lee's retreat from Gettysburg, and his work to erect a pontoon bridge to get back across the Potomac. The episode focused on Meade's decision not to test Lee's land defenses, and it left me wondering where the river fleet was during all this? Even if they didn't attack, why couldn't they just set a bunch of derelicts and debris adrift upstream, and home that the current smashes it into the pontoons? (I.e., like the Austrians did to Napoleon at the Battle of Wagram.)
Lee's army crossed the Potomac at Williamsport, Maryland. The Potomac is cut by quite a number of ledges, and there is no navigation upstream from the Chesapeake. In fact, the rough and rocky character of the river- and a series of rapids by Washington DC ( Great Falls) and what's now Harper's Ferry ( Bull Falls)- were the reason George Washington and some other investors would start the Potowmack Company in 1785, to make the river navigable to bateau traffic. The resulting series of cuts, sluices and gates were not very practical or profitable, and later replaced by the C&O Canal. The Canal would have been in operation in 1863, but it was far too narrow for a fleet of ships. So, the only large Union force Lee had to deal with came by land, and from the east. It was held up at the Battle of Funkstown.
Bridges were not always needed. In September 1862, on Lee's first big foray north, after encountering McClellan's very large army coming from the east he began retreating back across South Mountain and south towards a very important ford on the Potomac, Packhorse Ford at Shepherdstown. He didn't quite make it before McClellan arrived, and had to fight. That was the Battle of Antietam, after which Lee's troops were able to wade across the Ford to Virginia.