Base-level is an imaginary horizontal surface to which the fluvial system strives to erode. A distinction is made between a terminal and a local base-level. The base-level of many alluvial fans is an inland, local, intermountain basin. Its margins, which are the typical site of fan deposition, are not base-level loci. A falling base-level will trigger entrenchment only if the new exposed area will be steeper than the threshold gradient for erosion. Intersection points that are the end spot of an entrenched fan segment do not function as a base-level. Profile shortening by toe trimming causes headward entrenchment although not a base-level drop.