I took this photo from the airplane window. Why does this stream have this shape?
This is from the tidal marshes along the Georgia coast. As the tides rise and fall, the water flows in and out along these streams. Inertia causes the water to try to flow in a straight line, which means that most of the current is along the outside of each bend (also known as a meander.) The faster water of those currents erodes away sediments from the outside of the meander. The slower water on the inside of the bend drops any sediment it is carrying, building up the inside bank of the meander.
This process forms very twisty, windy streams. The pattern is very fractal, and I was reminded of blood vessels.