Grassy Island Creek is a tributary of the Lackawanna River in Lackawanna County, Pennsylvania, in the United States. It is approximately 4.4 miles (7.1 km) long and flows through Jefferson Township and Jessup. The watershed of the creek has an area of 5.42 square miles (14.0 km2). The lower reaches of the creek are impaired, but the upper reaches are not impaired. It is an intermittent stream during the summer, but its flow has been recorded as being as high as 116,553.21 gallons per minute. Some manganese, acidity, and alkalinity are also in the creek. It is in the Appalachian Mountain section of the ridge and valley physiographic province. The main rock formations in the creek's watershed are made of sandstone and interbedded sedimentary rocks. Some reservoirs are in the watershed at the