Darby Creek is a tributary of the Delaware River in Chester, Delaware and Philadelphia counties, in the U.S. Commonwealth of Pennsylvania. It is approximately long. The watershed of the creek has an area of . It has twelve named direct tributaries, including Cobbs Creek, Little Darby Creek, Ithan Creek, and Muckinipattis Creek. The creek has a low level of water quality for most of its length. The lower Darby Creek area was deemed a Superfund site by the U.S. Environmental Protection Agency (EPA) due to contamination with dangerous chemicals from two landfills.
Darby Creek flows through a narrow valley in its upper reaches and a tidal flat in its lower reaches. The creek is in the Piedmont Uplands and Atlantic Coastal Plain physiographic provinces. Major rock formations in the watershed include the Wissahickon Formation. Three small dams historically existed on the creek, but were removed in 2012. The watershed of the creek is largely developed, with roughly half a million people inhabiting it. Most of the watershed is in Delaware County, but some parts are in Chester County, Philadelphia County, and Montgomery County. The watershed is part of the Lower Delaware drainage basin.
Historically, the Lenni Lenape people inhabited the area in the vicinity of Darby Creek. By the 17th century, the Dutch and Swedish had arrived in the area, followed some years later by the English. Numerous mills of various types were eventually built along the creek and several railroads traversed the watershed. In modern times, grants by various organizations have been awarded to improve the creek and its watershed. The Darby Creek Valley Association operates within the watershed. Part of the creek's length is designated as a Coldwater Fishery and a Migratory Fishery, part is a Trout Stocked Fishery and a Migratory Fishery. Various species of fish, including redbreast sunfish, eels, and trout, inhabit it. Several areas in the vicinity of the creek are listed on the Delaware County Natural Areas Inventory. These include the Darby Creek Mouth Mudflat, the John Heinz National Wildlife Refuge, and the Ithan-Darby Creek Wetlands. A reach of the creek is navigable by canoe.
Course Darby Creek begins in a small valley near US Route 30 in Tredyffrin Township, Chester County. It flows south-southeast for more than a mile before entering Easttown Township and turning east-northeast for several tenths of a mile. The creek then turns southeast for a few miles, passing through Newtown Township, Delaware County and entering Radnor Township, where it receives its first two named tributaries, Thomas Run and Little Darby Creek, from the right and left, respectively. The creek then turns south-southeast for several tenths of a mile before turning southeast for several more miles. In this reach, it receives the tributary Miles Run from the left and the tributary Camp Run from the right. The creek eventually turns east briefly before receiving the tributary Ithan Creek from the left and turning south-southeast for several miles along the border between Marple Township and Haverford Township. In this reach, the creek flows alongside Interstate 476 and crosses it once. As it continues downstream alongside the highway, it crosses Pennsylvania Route 3 and receives the tributary Langford Run from the left. Near Pilgrim Gardens, it turns southeast again and begins to flow along the border between Upper Darby Township and Springfield Township, crossing US Route 1 and receiving the tributaries Colleen Brook and Lewis Run from the left and right, respectively. Further downstream, the creek's valley becomes much steeper and narrower and it makes several meanders as it passes by Clifton Heights and Lansdowne before flowing along the border between Aldan and Yeadon. The creek then flows south-southeast for a few miles, leaving its valley and passing Darby, Collingdale, Sharon Hill, and Colwyn as it crosses US Route 13. It then receives the tributary Cobbs Creek from the left.
The creek turns south and enters the John Heinz National Wildlife Refuge 4.5 miles upstream from its mouth. At this point it passes through the largest freshwater marsh in Pennsylvania before becoming entirely tidal. It turns west-southwest for a few miles, passing by Philadelphia, Folcroft, Norwood, Prospect Park, Tinicum Township, and Ridley Township and receives the tributary Muckinipattis Creek from the right. Darby Creek winds through a large lagoon 30 ft deep in places that was dredged in the 1960s. The creek then receives Stony Creek, its last named tributary, from the right and turns south for a few tenths of a mile before reaching its confluence with the Delaware River.
Darby Creek joins the Delaware River upriver of its mouth.
Tributaries Tributaries of Darby Creek include Cobbs Creek, Little Darby Creek, Ithan Creek, Muckinipattis Creek, and numerous others. Muckinipattis Creek joins Darby Creek upstream of its mouth and its watershed has an area of . Cobbs Creek joins Darby Creek upstream of its mouth and its watershed has an area of . Ithan Creek joins Darby Creek upstream of its mouth and its watershed has an area of . Little Darby Creek joins Darby Creek upstream of its mouth and its watershed has an area of .
Hydrology
For most of its length, Darby Creek has a low level of water quality. However, in some reaches of the watershed, especially the upper reaches, the streams in the watershed have a medium-low or even medium-high level of water quality. A Superfund site known as the Lower Darby Creek Superfund Site contaminates Darby Creek and impacts the creek's water quality. The creek is considered to be impaired for its lower . The causes of the impairment are habitat modification, siltation, and flow variability stemming from urban runoff and storm sewers.
Two measures of the instantaneous discharge of Darby Creek at Foxcroft in the 2000s were 10 and 17 cubic feet per second. The specific conductance of the creek ranged from 274 to 313 micro-siemens per centimeter. The pH was slightly alkaline, ranging from 7.3 to 7.8. The concentration of water hardness in the creek ranged between 100 and 109 milligrams per liter.
The concentration of dissolved oxygen in Darby Creek was measured to range from 7.4 to 12.4 milligrams per liter. The carbon dioxide concentration ranged from 1.9 to 3.4 milligrams per liter and the nitrogen concentration range from 2.1 to 2.2 milligrams per liter. The concentration of organic nitrogen ranged between < 0.19 and 0.25 milligrams per liter and the concentration of organic carbon ranged from 2.1 t to 2.2 milligrams per liter. The concentration of ammonia in filtered water was less than or equal to 0.02 milligrams per liter in two measurements and the nitrate concentration was half that in two measurements. The orthophosphate concentration in filtered water ranged from < 0.031 to 0.071 milligrams per liter and the phosphorus concentration in unfiltered water ranged from 0.032 to 0.045 milligrams per liter.
The concentration of magnesium in Darby Creek ranged from 10.7 to 11.7 milligrams per liter in filtered water and the concentration of calcium ranged from 22.5 to 24.6 milligrams per liter. The sodium concentration ranged between 11.1 and 13.0 milligrams per liter and the potassium concentration ranged between 2.15 and 2.80 milligrams per liter. The manganese concentration ranges from 10.6 to 25.6 milligrams per liter and the iron concentration ranges from 18.9 to 60.7 milligrams per liter. The boron concentration ranges from an estimated 12 to 16 milligrams per liter.
The concentration of fluoride in Darby Creek is less than 0.01 milligrams per liter and the chloride concentration ranges from 26.2 to 33.6 milligrams per liter. The sulfate concentration ranges between 18.1 and 19.9 milligrams per liter. The silica concentration was twice measured to be 15.7 milligrams per liter.
Geography, geology, and climate
The elevation near the mouth of Darby Creek is above sea level." The elevation of the creek's source is between above sea level. The gradient of the creek for its first is per . For the next , the gradient of the creek is per . For the last , the gradient is only a few feet per mile.
The valley of Darby Creek is considerably narrower in its upper reaches, but widens out as it approaches the Delaware River. Numerous reaches of the creek have been armored with boulders or riprap. Upstream of the tributary Cobbs Creek, the stream's channel is sinuous and the creek flows through a narrow valley bordered by low, steep hills. From Cobbs Creek downstream to its mouth, the creek is in a tidal flat.
The northern section of Darby Creek is within the Piedmont Uplands physiographic province, while the southern section is part of the Atlantic Coastal Plain province. The Piedmont Uplands section has generally old, hard upland rocks that eroded from the Appalachian Mountains. The rocks from the northern portion of the watershed date to the Precambrian Era and Lower Paleozoic Era. The rocks from the southern portion of the watershed are newer, dating from the Tertiary and Quaternary Periods. Atlantic Coastal Plain rock is generally softer than Piedmont Uplands rock and was deposited in the area about 1.6 million years ago through glacial erosion.
Several geologic formations can be found within watershed of Darby Creek. The Wissahickon Formation is the most prevalent formation in the watershed. It is derived from derived from sandstones and mudstones and is composed of mica schist. Metamorphic felsic gneiss and mafic gneiss formations are common in the northern parts of the watershed. The Bryn Mawr Formation and the Bridgeton Formation are also present and are unconsolidated deposits of rock that rest on top of the dense crystalline bedrock. Mica slate is present in Marple Township and was manufactured to form "Darby Creek scythe stones" in the 1860s.
Darby Creek tends to be a fast stream with some riffles. There is also whitewater in places. The creek is a "radically intermittent storm drain" in its upper reaches and a tidal estuary in its lower reaches. It passes through the only substantial tidal wetlands in Pennsylvania.
There used to be three small dams on Darby Creek. Going downstream, their heights were , , and . However, they were removed in late 2012 because they were contributing to flooding in the area and blocking fish passage.
During several measurements in the 2000s, the water temperature of Darby Creek at Foxcroft ranged from . The air temperature in the area ranged from . The barometric pressures range from 759 to 762 mm Hg. The average annual rate of precipitation in the watershed of Darby Creek is between .
Soil Three soil associations exist in the Darby Creek watershed. The Neshaminy-Lehigh-Glenlg soil association is prevalent in the northwest part of the watershed. It consists of silty, well drained, gravelly, and deep soil that rests on gabbro and granodiorite bedrock. The Chester-Glenlg-Manor soil association is prevalent throughout the watershed except in its lower reaches. It consists of silty, channery, and shallow to deep soil that rests on brown schist and gneiss bedrock. The Urban Land-Wetbrook-Pitts soil association is prevalent in the southern part of the watershed. It consists of silty, sandy, and deep soil that rests on coastal sediments. Roughly 53% of the land in the Darby Creek watershed is classified as having slightly erodible soil.
Watershed