PAK Diagram of a City, Is There More or Less Surface Water Runoff after Urbanization?

Peralta Colleges, Physical Geography
Rita D. Haberlin, Instructor


  1. Running water is the most important agent of geologic erosion shaping the landscape.  Surface water runoff takes the form of sheetwash, rill erosion, and streamflow. The water in streams comes from surface water runoff and groundwater (base flow).

  3. Over time, surface water runoff and stream flow combine to form a drainage system (a branched network of streams and slopes) that efficiently removes surface water runoff and sediment (material picked up by the stream). A drainage basin is the total area drained by a stream and its tributaries. A line dividing one drainage basin from another is called a divide. Most drainage basins are pear-shaped and have dendritic patterns that resemble the veins of a leaf. Base level is a limiting level below which a stream cannot erode. Sea level is the ultimate base level for most streams.

  5. The velocity of a stream determines the ability of a stream to erode, transport, and deposit sediment. The gradient (slope of a channel), channel shape and roughness, sediment load, and discharge (volume of water), all influence the velocity of a stream.

  7. Streams erode by hydraulic action (the sheer force of water), abrasion, and solution. A streamís load is transported in suspension (clay and silt), in solution, and by traction (rolling, dragging, or sliding of bed load along the bottom of the streambed). On average, about 20% of the load in U.S. streams is in solution.

  9. Over long periods of time, a stream channel becomes adjusted to the surface runoff and sediment load supplied by tributaries and the watershed. Downstream, a stream increases in width and depth, and discharge (volume of water). The load decreases in particle size and increases in quantity.

  11. Deposition occurs if the streamís velocity is slowed or its load increases. Suspended particles will settle out from a streamís load and form alluvium. Braided channels may develop in the streambed. An alluvial fan forms where a stream breaks out of a mountain canyon onto a flat plain slowing the velocity of the stream.

  13. Stream courses often show several stages in gradation from the mountains to the sea. The most obvious differences in valleys are the narrow V-shaped mountain valleys and the wide alluvial valleys closer to the sea. Narrow mountain valleys have a steep gradient and an irregular long profile of waterfalls, rapids, and lakes. Vertical erosion, the dominant process, produces steep-sided canyons in bedrock. Potholes form in the streambed when pebbles erode the valley floor by abrasion.

  15. In wide alluvial valleys, the supply of load increases while the streamís capacity to carry it increases. At this stage, meanders (S-shaped bends) cause lateral (side to side) erosion which widens the valley by eroding bluffs on the outside bend and depositing point bar deposits on the inside bend.
    Eventually, the steam flows freely across a wide floodplain. The material deposited by a stream in its floodplain is called alluvium. The alluvial stage is characterized by cutoffs, oxbow lakes, natural levees, yazoo streams (flowing parallel to the main river), and backswamps.

  17. Rivers carrying large quantities of sediment will eventually drop their load when they slow down and enter lakes or the ocean. This sediment forms a delta.

  19. Over a long period of time, the land is thought to be eroded down to a gently sloping surface called a peneplain. However, this series of stages in the development of landscapes may be interrupted in various ways. Rejuvenation in river valleys may take place if there is local uplift of the land, a lower sea level, increased rainfall, or a reduction in the load carried by a stream. The stream would then cut down into its own valley and produce terraces and meanders.

  21. Urbanization changes the proportion of surface runoff and base flow in streams. Impermeable surfaces such as concrete and asphalt reduce infiltration and thus the amount of baseflow available from groundwater. Impermeable surfaces also cause more rapid runoff and flash flooding.
    People's activities have greatly increased the suspended load of streams (e.g., cultivation, construction, and deforestation). Sediment load in U.S. streams has risen 2-1/2 to 10 times during the past two hundred years. Dam building and water diversions also affect the stream system by holding back sediment which increases erosion below the dam (e.g., Aswan Dam and the Nile valley, Egypt).

  23. Flooding may be reduced by reforestation, erosion control, and flood storage dams. Artificial levees and diversion basins may help hold back the stream at flood stage.

  25. Water quality in streams may be reduced by pollution from industrial wastes, acid mine drainage, agricultural wastes, sewage, radioactive wastes, and thermal outflows (heated water).


  1. Describe the forms taken by surface water runoff.
  2. Define the words:  drainage system, divide, drainage basin, dendritic pattern, and base level.
  3. List the factors that influence the velocity of a stream.
  4. Describe three ways streams erode and three ways they transport their load.
  5. Explain how a stream channel changes as it moves toward the sea.
  6. Describe the cause of deposition in river valleys and the landforms created by stream deposition.
  7. Describe the characteristics of the narrow mountain valley stage of a stream.
  8. Describe the characteristics of the wide alluvial stage of a stream.
  9. Define delta and peneplain.
  10. Describe the causes of rejuvenation in river valleys.
  11. Describe the impact of urbanization and people's activities on streams.
  12. Describe three methods of flood control.
  13. Describe six ways in which streams may become polluted.


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