Have any horseshoe lakes been found on Mars? Or have there been other signs of rivers persisting long enough to form altered channels? For the purposes of this question, I would exclude multiple channels such as those found in deltas.
This Arizona State University website Channel Flow Features has a writeup on this
A channel that meanders (winds back and forth) is a common feature of terrestrial streams, and Martian channels follow the pattern also. Anyone who has canoed, kayaked, or inner-tubed down such a stream has personally experienced the characteristic features of a meandering channel.
At each bend, the water flows fastest on the outside. This undermines the stream bank, and deepens the channel bed there. On the inside of the bend, the flow runs slowest, and mud, sand, and gravel settle out and accumulate to make a shallow bar. Undermining the bank on the outside of the bend causes the meander to move downstream.
Eventually the erosion will cut through to the next bend, and the stream briefly straightens its path. On Earth, the cutoff portions of the channel become curving "ox-bow" lakes. As many years pass, these fill up with windblown dust and silt and vegetation, slowly becoming swamps, marshes, and then meadows.
On Mars, meandering streams generally do not show the full development of shapes seen on Earth. This is probably because the Martian streams did not flow for long enough. The closest Mars comes to having ox-bow lakes are cutoff portions of winding channels in deltas that formed in crater lakes.