Looking north from about 79th Street. This is the original Riverside Park conceived by Frederick Law Olmstead in the 1870s and completed in 1910 (click on the small picture at left). Except for the terraced part (center of picture) adjacent to Riverside Drive it slopes steeply down towards the New York Central railroad tracks and Hudson River, and in any case extended northwards only to 125th Street. The Great Depression was still in the future, so the Park is in good condition, although whatever trees had been planted are still just saplings. Nevertheless it is clear that public access to the River is blocked by the railroad and the parkland below the terrace is not very useful. I have no idea what the buildings are west of the tracks or how they were accessed.
Once the Depression set in four years later, the park deteriorated and homeless people built shanties along the riverbank. Robert Moses' idea was to rip up the railroad tracks, move them over towards Riverside Drive, and cover them with flat parkland. Then alongside them a lower level of the park, also relatively flat, would be separated from the upper level by a thick stone retaining wall, with stairways every few blocks. The lower level would be expanded into the river by adding fill, and a highway would be built close to the river's edge, with a ribbon of parkland between the highway and the riverbank, accessible through a series of highway underpasses.
Moses' plan was called the West Side Improvement, and it was realized with New Deal designers, architects, engineers, and workers. In 1934-37, everything you see in the photo to the left (west) of Riverside Drive was gouged out and replaced with a whole new park, the one we have today.
Today's park has three levels: street level (similar to the terrace in the picture, but with a constant width rather than bulges and narrows), a mid level covering the train tracks, and a lower level alongside the Henry Hudson Parkway and the River. It was an almost inconceivably massive earth-moving and construction project that required four years and unknown thousands of workers. And unlike the old park, it is full of playgrounds, athletic fields, and game courts.