The is what Wikipedia has to say about levees... "Levees are commonly thought of as man-made, but they can also be natural. The ability of a river to carry sediments varies very strongly with its speed. When a river floods over its banks, the water spreads out, slows down, and deposits its load of sediment. Over time, the river's banks are built up above the level of the rest of the floodplain. The resulting ridges are called natural levees. When the river is not in flood state it may deposit material within its channel, raising its level. The combination can raise not just the surface, but even the bottom of the river above the surrounding country. Natural levees are especially noted on the Yellow River in China near the sea where oceangoing ships appear to sail high above the plain on the elevated river. Natural levees are a common feature of all meandering rivers in the world."

However, we should also take note that according to another article, "A natural levee is a narrow ridge of alluvium deposited at the side of the channel. During high discharge periods when the stream floods, coarse sediment settles out near the stream channel and grades to finer material further away. The over bank deposits of alluvium are often rich sources of nutrients for soils developed on the floodplain. Because floodplain soils are usually quite fertile, humans have inhabited them for years. To prevent flooding, artificial levees are built close to the channel, typically higher than natural levees. Confining the flood discharge to a small area increases the velocity of flow.

Above, returning to the earlier photo, a fine channel (at one time) drew in water from the rio Paraguay at the northern side, traverses the site and exits on the southern side.