Caddisflies are emerging from the streambed in small numbers — a promising sign, since the larvae of these insects are exquisitely sensitive to water quality and require clean gravel and adequate dissolved oxygen to develop. The stream here is holding steady at 8 mg/L, cool and borderline tolerable. Their presence, even in modest counts, suggests the system is not yet moribund.

Yet this resilience masks a much darker picture. The Navy Yard upstream has discharged polycyclic aromatic hydrocarbons (PAHs)—oily, persistent toxins—at nearly 360 times the permitted limit in two consecutive quarters. PAHs bioaccumulate in stream sediments and organisms; even at sub-lethal doses, they impair reproduction and immune function in invertebrates. The caddisflies here are not a sign of recovery. They are scouts in a landscape under siege, persisting in the narrow margin between chronic poisoning and collapse.