What is the term for a stagnant lake that forms along a winding river due to soil erosion resulting in an abandoned stream, cut off from the river?

An oxbow lake  is the term

An oxbow lake is a stagnant lake that forms along a winding river due to soil erosion resulting in an abandoned stream, cut off from the river. An oxbow lake begins as meander or a curve in a river after some time, the curve becomes larger as a result of erosion and hydraulic action. Oxbow lakes are commonly formed in flat, low-lying plains that is not far from where the river flows into another water body. The curve that forms an oxbow lake consists of the inner and the outer river banks.  

i think the answer to your question is: settlement of the interior and economic growth. ( ^_^)／