Why isn't the District of Columbia square?

If you've ever looked at a map of Maryland and Virginia and wondered why the otherwise regular boundaries of the District of Columbia only enclose land on the Maryland side of the Potomac, you aren't alone. I, too, found this truncation curious, as the original Residence Act passed in 1790 stipulated a district that was to be ten miles by ten miles square. It turns out, this was originally the case. When the Organic Act of 1801 placed the newly delineated District of Columbia under the exclusive purview of Congress, it included the new federal city of Washington, the existing city of Georgetown, the existing city of Alexandria, and many acres of rural land.

The map above, though from much later, shows how the District's boundaries would've looked in 1801. Since Washington did not undergo a major period of population until after the Civil War, the region was quite rural and sparsely populated for much of the Nineteenth Century. Throughout the Antebellum period, government of the District was split between the three city governments of Washington, Georgetown, and Alexandria (which were referred to as Georgetown, DC, and Alexandria, DC, at that time), and the two county governments of Washington County on the north side of the Potomac and Alexandria County (later, Arlington County) on the south side of the Potomac.

As the Nineteenth Century sped toward the Civil War, the Virginia side of the District began agitating to be freed of federal domination and return to Virginia. This was partly because Alexandria had lost much of its share of the trading economy to Georgetown, and partly because there were rumors that slavery was to be outlawed in the District. Thus, in 1846, the retrocession of Alexandria and Alexandria County to Virginia was completed. This left the District of Columbia with land only on the Maryland side of the Potomac.

Eventually, due to the rapid growth of the capital after the Civil War, the population growth was such that the city of Washington annexed all of the County of Washington, and eventually took Georgetown too. This process was completed in 1871, when a the District was consolidated into a single local government unit - an arrangement that exists to this day. When Georgetown was absorbed into the consolidated government, all of the streets were renamed to match the existing conventions present throughout the rest of Washington. Thus, though the streets are much older, the main thoroughfares have rather modern names. M Street and Wisconsin Avenue, the two major thoroughfares, were originally named Bridge Street and High Street, respectively.