Grassy Island Creek is a tributary of the Lackawanna River in Lackawanna County, Pennsylvania, in the United States. It is approximately long and flows through Jefferson Township and Jessup. The watershed of the creek has an area of . 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 boundary between the Pocono Formation and the Llewellyn Formation.
The watershed of Grassy Island Creek is mainly forested, but there are also residential lands and abandoned mine lands. The creek is a source of flooding in Jessup. The remains of collieries such as the Sterrick Creek Colliery, the Mount Jessup Colliery, and the Pompey Colliery also occur in its vicinity. In 1996, the creek's channel failed, causing it to wash more than 30,000 cubic yards of culm into the Lackawanna River. In the 2000s, various channel repair efforts were carried out. A proposed natural gas-fired power plant intends to dump waste water into the creek.
Course
Grassy Island Creek begins on a mountain in Jefferson Township. It flows southwest for several tenths of a mile, until it reaches a valley. In the valley, the creek turns northwest for more than a mile, receiving two unnamed tributaries from the left and entering Jessup. In Jessup, it turns south-southwest for a few tenths of a mile and enters the Olyphant Reservoir Number Two, where it receives an unnamed tributary from the right. From the southern side of the reservoir, the creek turns west-southwest for a short distance, receiving an unnamed tributary from the left and entering the Olyphant Reservoir Number One. From this reservoir, the creek flows northwest for several tenths of a mile, receiving an unnamed tributary from the left. It eventually turns west and crosses US Route 6. Soon afterwards, it leaves the valley and turns north. Several tenths of a mile further downstream, the creek reaches its confluence with the Lackawanna River.
Grassy Island Creek joins the Lackawanna River upriver of its mouth.
Tributaries
Although Grassy Island Creek has no named tributaries, has several unnamed ones: "Unnamed trib 1", "trib from Olyphant Res. No. 3", "Unnamed trib 2", and "Unnamed trib 3". Their lengths are , , , and , respectively.
Hydrology
The lower reaches of Grassy Island Creek are designated as impaired, although the upper reaches are unimpaired. The causes of the impairment include pH and metals. The source of the impairment is abandoned mine drainage. An unnamed tributary is impaired, with a cause of flow alterations and a source of abandoned mine drainage.
In a 2002 study, the average discharge of Grassy Island Creek at its mouth was 6011.296 gallons per minute. The discharge ranged from 261.22 to 16,553.21 gallons per minute. However, during the heat of summer, it is an intermittent stream. A 1952 report estimated that the rate of surface seepage into mine workings was 8.35 gallons per minute per inch of rain. The report also estimated that the rate of streambed seepage into mine workings was 17.86 gallons per minute per inch of rain. In 2002, the pH of the creek ranged from 4.8 to 5.5, with an average of 5.05.
The load of manganese in Grassy Island Creek is per day and the manganese concentration is 0.06 milligrams per liter. It meets its total maximum daily load requirements. Iron and aluminum were not detected in the creek during its total maximum daily load study. However, the daily load of acidity is , considerably higher than the total maximum daily load of per day. The average acidity concentration is 13.23 milligrams per liter and the concentration ranges from 8 to 18 milligrams per liter. The concentration of alkalinity in the creek averages 7.33 milligrams per liter and the daily load of alkalinity is . The alkalinity concentration ranges from 6.4 to 9.4 milligrams per liter.