The theoretically lowest level to which the course of a river can cut down. This level may be sea level, the junction between a tributary and the main river, or the level of a waterfall or lake, but streams rarely erode as far as base level. Base level may alter due to eustatic or isostatic change, and be termed positive changes of base level if the land sinks relative to the sea, or negative changes of base level if it rises. (Try Schumm (1992) J. Geol. 101 on river response to base level change; see also rejuvenation). Marine base level is the lowest point at which marine erosion occurs, perhaps as low as 180 m below the surface.