The Delaware River is a 94-mile-long (151 km) river in the northeastern part of the state of Kansas. The Delaware River basin drains 1,117 square miles (2,890 km2) from the water of Perry Lake reservoir. The river is a Category 1 watershed by the Kansas Department of Health and Environment. This means that the watershed needs to be restored and protected. The river is one of the major tributaries of the Kansas River.