Frame an area of a world, then open it up close to shape it.
The yellow square follows the middle of the view: move and zoom to frame the area.
⌘-drag (Ctrl-drag) on the land to shape it with the brush.
Rivers and lakes follow your shaping when you update them, or when you go back to the world.
Then drag from one point of the ground to another: the distance, and how much it rises or falls. Esc clears it.
Shaping is a draft, kept in this browser: the world itself changes only when it's applied. Red squares are where players have built: shaping keeps clear of them.