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