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.