It is a mix of both, but the stagnant design creates the perfect trap. While sewage spills introduce the initial organic matter, the canal's physical layout dictates whether that matter stays safe or becomes deadly.
Think of it this way: sewage provides the fuel, but the lack of flow provides the incubator. When runoff enters the waterway, it brings nutrients and bacteria. In a healthy, fast-moving river, these materials disperse quickly. However, Brentford Lock suffers from poor circulation. This lack of movement creates anaerobic, or oxygen-free, pockets in the silt and sludge at the bottom.
Under these specific conditions, *Clostridium botulinum* thrives. This bacterium loves low-oxygen environments where it can produce toxins. Even if sewage levels dropped tomorrow, the existing buildup of fine sediment and the restricted water turnover would still present a risk. The design limits the canal's ability to self-clean through natural flushing. Consequently, the stagnant water acts more like a series of disconnected ponds than a flowing artery. You end up with localized zones of high toxicity that persist long after a specific contamination event has passed.
