Big Blue River south of Tuttle Creek Lake, courtesy Wikipedia.

The Big Blue River, the largest tributary of the Kansas River, flows for approximately 250 miles from central Nebraska into Kansas, where it joins the Kansas River east of Manhattan.

One of the principal water courses of northeastern Kansas, it comprises two branches. The north fork rises in Hamilton County, Nebraska, and the south fork in Adams County, Nebraska. They unite near the town of Crete, from which the mainstream flows south, passing through the western part of Marshall County, Kansas, forming the boundary between Riley and Pottawatomie Counties, and emptying into the Republican River at Manhattan. It was named by the Kanza Indian tribe, who camped at its mouth from 1780 to 1830.

As it passes through Nebraska and Kansas, it mostly crosses agricultural land. Shortly before intersecting with the Kansas River, the Big Blue discharges its waters into a reservoir called Tuttle Creek Lake near Manhattan. The land surrounding the reservoir is part of a state park.