TORONTO REGION CONSERVATION AUTHORITY By

Urban development alters the composition of surface materials by introducing significant areas of asphalt, concrete and other impervious surfaces over previously porous soils. The impervious materials prevent rainfall from soaking into the ground thereby increasing the amount or volume of rainfall that flows into nearby watercourses. In addition the drainage system is enhanced through swales, curbs and gutters, catchbasins and stormsewers such that these flows are drained as quickly as possible to the watercourse. The result is an increase in flow magnitude and duration in the watercourse. This impact is referred to as hydromodification. Commensurate with the change in flow rate and volume, urbanization results in alteration of the physical characteristics and mass of sediments delivered to the watercourse. Following disruption of the soils during the active construction phase, which results in an increase in sediment inputs to the watercourse, the landscape is stabilized as lawns or paved areas are established and the sediment yield declines significantly relative to pre-development agricultural conditions. This impact is referred to as alteration of the sediment-regime. The quality of stormwater runoff is also altered by urban development and the human activities that take place in urban areas which contribute to the pollution of runoff and the resulting degradation o