This is looking west from the Washington Monument at West Potomac Park in 1912.
Almost none of the National Mall west of the Washington Monument grounds (the road running across the bottom of this image) and south of Constitution Avenue NW (the road on the right right of this image) existed prior to 1882. It was all part of the Potomac River. A low tide, there might be only a few inches of water there. When the river ran low in the fall or dry weather, it might actually be marshy or dry land.
After terrible flooding inundated much of downtown Washington, D.C., in 1881, Congress ordered the Army Corps of Engineers to dredge a deep channel in the Potomac and use the material to fill in this area. There really was a vast amount of material... This "reclamation" project created the current banks of the Potomac River and raised much of the land near the White House and along Pennsylvania Avenue NW by nearly 6 feet. The reclamation project -- which also built East Potomac Park out of nothing -- was largely complete by 1890. Initially, city boosters wanted the land to be used for industrial purposes. But smarter heads won out, and this land was designated Potomac Park by Congress in 1897.
It remained a dirt field full of weeds until Congress appropriated money for its beautification in 1902. This led to the planting of sod, bushes, and trees; grading and paving of sidewalks, bridle paths, and driveways; and the installation of irrigation, drainage, and sewer pipes.
Here's an altered image, showing where the Lincoln Memorial (begun in 1914 and completed in 1922) and the National World War II Memorial (begun in 2001 and complete in 2004) are today.