Levees form as rivers rise over their banks for a very simple reason: the current slows at the water's edge, therefore part of the sediment load in the water is dropped on the banks. Over many floods, this process builds up a gentle rise (the word comes from the French levée, raised). When humans come to inhabit a river valley, they invariably fortify the levee and raise it higher. Thus geologists take pains to specify a "natural levee" when they find one. The levees in this picture, in Transylvania, Romania, may have an artificial component, but they are typical of natural levees—low and gentle.
Levees also form underwater, in submarine canyons. There the sediment flows, or turbidity currents, going down the canyon form natural levees by the same mechanism as river levees.