Estuaries are found wherever rivers meet the sea. Salt marshes and mangrove forests are concentrated along low-lying coasts with sandy shores and may, like estuaries, be associated with the mouths of rivers. All three are at the transition between one environment and another-salt marshes and mangrove forests at the transition between land and sea, and estuaries at the transition between rivers and the sea. Because these areas are transitions between very different environments, they have a great deal in common physically, chemically, and biologically. These are environments that pulse to the rhythm of lunar-driven tides and teem with life. Figure 3.20 shows a rich salt marsh landscape, and figure 3.21 showe the structurally complex environments provided by dense populations of mangroves and their many prop roots.