Insequent streams have an almost random drainage often forming dendritic patterns. These are typically tributaries and have developed by a headward erosion on a horizontally stratified belt or on homogeneous rocks. These streams follow courses that apparently were not controlled by the original slope of the surface, its structure or the type of rock.

The word ‘insequent’ refers to stream forms along with:

Consequent streams are streams whose course is a direct consequence of the original slope of the surface upon which it developed, i.e., streams that follow slope of the original land.
Subsequent streams are streams whose course has been determined by selective headward erosion along weak strata. These streams have generally developed after the original stream. Subsequent streams developed independently of the original relief of the land and generally follow paths determined by the weak rock belts.
Resequent streams are streams whose course follows the original relief, but at a lower level than the original slope (e.g., flows down a course determined by the underlying strata in the same direction). These streams develop later and are generally a tributary to a subsequent stream.
Obsequent streams are streams flowing in the opposite direction of the consequent drainage.

The Insequent are a race of people new to The Last Chronicles (although they have been around a long time (and in some cases before)). Wayfriend on the SRD forum Kevin’s Watch interprets the name thusly:

Let’s start with Insequent. Insequent is actually a real word. It means “random”, in the sense of random-access: not predicated by a cause. It is the opposite of “subsequent” or “consequent”. Insequential means “belonging to no particular sequence”.

Sources: Wikipedia and Kevin’s Watch.