Usually this occurs where the stream crosses rock layers harder than downstream rocks. In time as the stream's gradient lessens, the stream matures and eliminates waterfalls and rapids and meanders. Through centrifugal force the curves of the stream widen. As the stream gets old , the valley bottom becomes broad as the stream meanders in great curves. The lowest level to which a stream may erode its valley is base level .
Sea level is the base level for rivers flowing into the sea.
A tributary can reach the base level of the main stream. Every major stream controls the base level of the streams flowing into it.