Why might herbicides in water ditches or streams move downstream?

The movement of herbicides in water ditches or streams predominantly occurs due to the arrival of water from surrounding areas, such as during a storm or when a lake overflows. This is because the increased water flow can transport dissolved substances, including herbicides, downstream. When rainfall or runoff enters a waterway, it can carry with it various pollutants that have accumulated in the water. This increased volume and velocity of water effectively dilute and disperse these chemicals, moving them further along the waterway and potentially impacting areas downstream.

While other factors may influence water quality or chemical presence, they do not directly lead to the movement of herbicides downstream in the same manner. For instance, a drought period might reduce water volume and limit movement, while the introduction of foreign chemicals could refer to entirely different pollutants or contaminants. Seasonal changes in aquatic life do not directly relate to the transportation of herbicides within the water. Thus, the influx of water from storms or other sources plays a crucial role in transporting herbicides downstream.