Gravity is essential to a stream's ability to erode and deposit materials. In fact, you can start by saying quite literally that streams would not exist without gravity. Gravity is the reason water falls from the sky, and, once it has fallen, gravity pulls water to the lowest elevation. Gravity creates streams. Gravity is the reason water continues to flow into streams, and the reason water flows along streams to rivers and lakes. Since it the motion of water along, over, and through things that causes them to erode, gravity is the force driving erosion. Gravity is erosion's engine. The process of depositing materials is known as sedimentation. Gravity is one of the forces shaping sedimentation. Other factors also apply, like the viscosity of the liquid in which particles are suspended. In general, water's buoyancy pushes the particles up, in suspension, and gravity pulls them down, so they are deposited.