The Trinity's pollution in North Texas comes largely from nonindustrial sources, such as sewage treatment plants and urban runoff - water flushed into the river from streets and creeks with each rain.

The Trinity River, particularly the segment flowing through the Dallas-Fort Worth metroplex, has long struggled with a "gross" reputation due to urban runoff, industrial discharge, and historical neglect. Because it serves as the primary drainage basin for a massive concrete jungle, the river collects high concentrations of "non-point source pollution," including motor oil, pesticides, and litter washed off city streets during heavy North Texas rainstorms. Historically, the river was used as an unofficial disposal site for industrial waste, leading to high levels of PCBs and heavy metals in the sediment. Additionally, the river suffers from low flow rates during the hot Texas summers, which can lead to stagnant water and "algae blooms" that contribute to a murky appearance and unpleasant odors. While 2026 cleanup initiatives and the "Trinity River Vision" project have significantly improved water quality for wildlife, the high bacteria counts (specifically E. coli) from sewage overflows and animal waste mean that "contact recreation" like swimming remains strictly prohibited.