In 1996, the creek's channel failed, causing it to wash more than 30,000 cubic yards of culm into the Lackawanna River.

A proposed natural gas-fired power plant intends to dump waste water into the creek.

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 this reservoir, the creek flows northwest for several tenths of a mile, receiving an unnamed tributary from the left.

Several tenths of a mile further downstream, the creek reaches its confluence with the Lackawanna River.

[1] Grassy Island Creek joins the Lackawanna River 20.36 miles (32.77 km) upriver of its mouth.

An unnamed tributary is impaired, with a cause of flow alterations and a source of abandoned mine drainage.

[4] In a 2002 study, the average discharge of Grassy Island Creek at its mouth was 6011.296 gallons per minute.

[5] 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.

[8] The peak annual discharge of Grassy Island Creek at its mouth has a 10 percent chance of reaching 550 cubic feet per second.

[9] Leeward Construction once requested a permit to discharge stormwater associated with construction activities into Grassy Island Creek, as did Seefried Properties, Inc.[10][11] The elevation near the mouth of Grassy Island Creek is 830 feet (250 m) above sea level.

[1] Grassy Island Creek is in the Appalachian Mountain section of the ridge and valley pysiographic province.

In some reaches, the creek flows through steep ravines that are owned by the Theta Corporation, the Pennsylvania Game Commission, and The Nature Conservancy, as of 2001.

Downstream of the Olyphant Reservoir Number One, the creek becomes significantly impacted by historic anthracite mining.

[6] A culm dump known as the Rose Pile is situated on the floodplain at the mouth of Grassy Island Creek.

The creek passes through the Robert Casey Highway via a 400-foot concrete box culvert through a mound of fill, but reappears on the surface at Sunnyside Road.

The Olyphant Reservoir Number Two was breached by the Pennsylvania Gas and Water Company in 1994, partly for dam safety reasons.

This failure contributed to a washout of the Rose Pile, which caused more than 30,000 cubic yards of culm to erode into the Lackawanna River.

[3] In the early 2000s, a trail project known as the Grassy Island Creek Greenway was proposed in the Lackawanna River Watershed Conservation Plan.

The project involved riparian and drainage enhancement, as well as surface reclamation, along 1 mile (1.6 km) of the creek and over an area of approximately 400 acres (160 ha).

In 2001, the channel of the creek at its mouth was relocated in a project carried out by the Lackawanna County Conservation District.

[4] In 2015, a proposed natural gas power plant drew opposition for planning to discharge waste in to Grassy Island Creek.

[13] However, proponents of the power plant, such as conservationist Charlie Charlesworth, pointed out that this would also give the creek a constant flow.