Some reduction in dissolved oxygen occurs naturally as water flows downstream, but human activity has a marked effect. Water quality was degraded as the water moved downstream through agricultural and urban land. At Site 2, water quality was slightly degraded by an urban area. At Site 4, at the mouth of a tributary that drains intensively farmed land, levels of dissolved oxygen were extremely low and some species of fish could not survive. Figure 7-1 Dissolved oxygen in a B.C. stream A growing concern for stream and river quality is the entry of animal manure.
Fish sometimes suffer from the high ammonia levels in water contaminated by manure.
Over a longer period, bacteria consume available oxygen as the manure breaks down, and fish may suffocate. Nutrients in the manure also promote the growth of algae and aquatic vascular plants, which further deplete oxygen levels when they die and decompose. Streams bordered by shrubs and trees tend to be narrow and deep, whereas eroded streams become wider and shallower. Soil erosion adds to the sediment loading of streams and rivers and contributes substances that are attached to soil particles, such as nutrients, pesticides, and bacteria. Some fish depend on specific habitats to spawn.