In geology, the lowest level to which a river or stream can erode the land, usually at sea level or the level of its base of erosion.
From base + level (from Old French livel, from Latin libella meaning balance or measuring instrument). This geological term emerged in 19th-century earth science to describe fundamental erosion concepts.
Baselevel explains why rivers can't carve infinitely deep—they have a natural stopping point, usually the ocean, which means the Grand Canyon won't keep getting deeper forever because it's already approaching its baselevel of erosion.
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