The patterns are the result of old river beds. Over time, rivers meander, eroding away on one side, and depositing on the other. This causes rivers to wind back and forth, creating loops. If erosion cuts through a loop, it gets cut off, forming an oxbow lake. Gradually the oxbow fills in with plant material, leaving a curve of very fertile soil, surrounded by ribbons of sand and gravel deposited by the river. The vegetation in this old oxbow tends to be much greener and more lush than the surrounding plants, letting you easily see where the river used to be.