An oxbow lake is where a U shaped still water lake exists alongside a flowing river.
When a river starts to meander, it can form an oxbow lake. As the river continues to meander its curves become more exaggerated as the water deposits silt on one side and erodes on the other. Eventually the curves become so close that a flood causes a breach to flow in the direction of least resistance and results in dropping sediment to form the lake.
The swirls and curves of the previous courses of the river in the Yamal Peninsula, Russia are easily seen from this aircraft photo. The lakes are generally formed when the river reaches a low lying plain where the escape to the sea is less exaggerated.
Around the area of a river bend, deposits occur on the convex bank (the bank with the lesser radius). Conversely, erosion and undercutting occur on the concave bank (the bank with the bigger radius.)