Urbanisation activities

Urbanisation has changed the face of many waterways that flow through urban areas

Urbanisation has changed the face of many waterways that flow through urban areas.

Land development

Many urban streams have been modified through installation of instream barriers (weirs, culverts, and pipes), land development and clearance, and removal of surrounding riparian vegetation. Land development involves the construction of new infrastructure, such as roads and buildings, and includes extensive earthworks that often lead to an increase in sediment, nutrients, and chemical contaminants that are washed into waterways from surface and stormwater runoff, especially after heavy rain. Urbanisation of a catchment changes the flow of the water because the natural processes of water delivery from land to a waterway are changed. Typically, water flushes off hard urban surfaces much faster and in greater quantities than natural ground surfaces, causing erosion and habitat change in streams. Construction of instream barriers can also change the level, speed, and direction of water flow.

Waste water

Historically, lakes and rivers close to urban centres may have received raw or partially treated sewage from domestic and industrial plants which contained wastes including pathogens and chemical contaminants. Some of these pollutants may still be retained in lake sediments. Today, sewage treatment plants provide both primary and secondary treatment of effluent from domestic and industrial plants. Most solids and pathogens are removed from the water through a series of ponds and the remaining effluent is discharged to the sea or a river.

Transport infrastructure

Transport infrastructure covers up to 30% of land in modern cities. These hard impervious areas prevent rainwater from soaking into the ground, causing far more runoff of sediment and chemical contaminants than occurs naturally, including large quantities of polycyclic aromatic hydrocarbons (PAHs) that, amongst other things, are a consequence of oil and petrol spills. These contaminants enter waterways through urban stormwater pipes or runoff during heavy rain.