Memories are like river because they flow, twist and turn, and keep running on through your head. The river flow is your thought process and your memories coming back to you. Sometimes you can watch the river or memories swim by for a long time but eventually you will lose track of them. They keep flowing down stream. If they go down to far stream then you will lose it forever. Some memories wont go down stream so fast though and you will be able to hold on to them for longer. They may even get stuck in the middle of the stream and you will be able to remember them forever.
Also memories keep on coming at you. The stream keeps flowing and the memories keep coming and going. The river is a cycle with no start and no end. Your memories may have a start but it is really vague. And the end you cant tell where it ends.