The Delaware River flows into the Kansas River, from the north, just below the small town of Perry, Kansas. The confluence is primarily of interest to recreational boaters, for whom the trip down the Delaware to the Kansas then down the Kansas to an impoundment at Lawrence makes a pleasant day trip. It is the arrival of the Delaware from the north that is geologically noteworthy, and illustrative of an interesting pattern of the Kansas River watershed.

The Kansas river lies along the southern edge of its watershed. All of its large tributaries, particularly along the lower stretch of the river, flow in from the north. The only meaningful exception is the Wakarusa River, which flows parallel to the Kansas before making a short jog to join the larger river. The Wakarusa flows so close to the Kansas that it is thought to have been an alternate channel when the more northerly channel was blocked by glacial ice.

The asymmetrical pattern of tributaries to the Kansas River results from the history of glaciation of the region. At the height of the Pleistocene glaciers, the southern edge of the ice sheets stopped immediately adjacent to the course of the modern Kansas River. The stopping point can be inferred by deposits of coarse glacial material and out-of-place boulders, called “erratics” along the Kansas River valley. As those same glaciers melted, that meltwater formed streams that flowed south from the leading edge of the retreating glaciers toward the Kansas River. Those streams were the ancestors to the Delaware and the other south-flowing modern tributaries to the Kansas River.