Adam C. Simon Ph.D., University of Maryland, 2003 Research Associate Department of Geology

1. Adam C. Simon Ph.D., University of Maryland, 2003 Research Associate Department of Geology University of Maryland College Park, MD 20742 p: 301 405 0235 f: 301 314 9661 e-mail: asimon@geol.umd.edu

2. The flow of fresh water in channels on the Earth’s surface has been essential to the development of topography and most ecosystems. Stream Flow Some of the consequences of natural stream flow present engineering and social challenges with which we grapple year after year, and have through civilization’s history.

3. Although water in streams and lakes is not significant in terms of the global water supply, it is a significant component of the fresh water budget. Flowing surface water is critical in erosion and dispersal of sediment. Global Water Budget p.254

4. Water Moving Sediment on Mars High resolution images of the Martian surface further demonstrate that running water was once present on the red planet’s surface.

5. The Hydrologic Cycle

6. Development of Stream Channels Once moving water forms a channel, the channel erodes upslope: headward erosion. The pattern of the small streams (tributaries) that feed the main stream is based on the material being eroded. Fig. 17.3

7. The ability of a stream to erode is based on velocity of water. Velocity is proportional to slope. Base Level At a sufficiently low slope, streams will run without eroding: this is called the base level. The ultimate base level is sea level, although local base levels exist, since flow paths are seldom one consistent slope.

8. Drainage Basin and Longitudinal Profile Fig. 17.5

9. Change in Profile = Change in Base Level Fig. 17.18

10. lower base level = steeper profile; increase stream erosion (down cut, incise) Base Level Changes raise base level = shallower profile; deposit sediment Processes involving base level decrease: -- sea level drop -- subsidence at mouth -- uplift at head of stream -- draining of lakes Processes involving base level increase: -- sea level rise -- uplift at mouth of stream -- subsidence at head -- creation of lakes

11. Base level is constantly in flux, naturally and artificially. Dams create artificial local base levels, with shallower slopes (sediment deposited) upstream and steeper slopes (erosion) downstream. Damming

12. Water Dynamics Stream sediment moves by rolling along the bed or by bouncing (saltation).

13. Stream Transport and Deposition 200 cm/s ~ 4.5 mi/hr

14. Processes by which streams erode include: Stream Erosion • scouring • abrading • lifting/breaking • dissolution

15. Darcy’s Law and Discharge If discharge increases and area is unchanged, velocity must increase. discharge = area x velocity

16. Internal Features of Stream Channels Turbulent water flow leads to the classic meandering path. In this configuration, water velocity through the channel is not equal.

17. Meandering Streams

18. Immature Streams Streams draining areas of high slope tend to have such heavy sediment loads and variable flows that they do not have the chance to develop organized meandering channels. Instead they are braided streams. By definition they are not long-lived.

19. Braided Streams braided stream typical of those draining melting glaciers in Alaska Upper Brahmaputra river, India, draining the immense Himalayan mountains

20. Alluvial Fans Alluvial fans occur in areas where steep slopes meet regions of essentially no slope. This causes an immediate deposition of all sediment, resulting in the characteristic fan shape.

21. By fundamental principle, when a stream reaches a large body of water and velocity drops, the material being transported is deposited. Deltas The sediment deposited here is called a delta. The coarsest-grained material drops out immediately, and finer grained sediment carries out slightly farther, creating a characteristic deposit.

22. Distributaries in deltas periodically shift to follow the most efficient path to the sea -- that with the steepest slope. This is not allowed to happen in today’s heavily-engineered Mississippi delta. Modern Delta Sediment Delta-related wetlands are under stress from sediment starvation and lack of natural flooding. p.302

23. Floodplains Floodplains form in basins carved from unconsolidated sediment or soft rocks. The floodplain doesn’t meander like the channel: it is the broad area of uniform topography around the stream.

24. Stream Terraces Fig. 17.21 Like lake terraces and coastal terraces, these indicate earlier floodplain positions for streams which erode their floodplains due to changes in base level.

25. Streams Locked in Valleys Streams like these have virtually no (ordinary) floodplains. They have carved into rock so deeply, that their meandering and other characteristic evolutionary features are restricted. Why do some streams erode bedrock like this?

26. Antecedent Streams uplift/compressive deformation

27. Superposed Streams Without knowledge of the regional geologic history, differentiating between an antecedent and superposed streams can be difficult.

28. Drainage Pattern Evolution Old superposed channels cut across regional structures. Younger channels follow predictable paths through easily eroded valley rocks.

29. Sedimentary rocks with sedimentary structures that show paleocurrent direction of streams give us further supporting evidence for plate movement, as in the evolution of drainage in Africa and South America in the last 300 Myr. Drainage Pattern Evolution

30. Watersheds When one drainage basin erodes into another, it is stream piracy. Every stream is defined by an area on the ground where incident precipitation will all flow into that stream. These drainage basins or watersheds are separated by topographic highs: divides.

31. In North America, the continental divide separates watersheds that drain to the west from those that drain to the east (and the arctic). The Continental Divide The Mississippi and its tributaries dominate the North American drainage system.

32. Western Hemisphere Drainage What happens to water in these regions? Most of stream water in the US drains into the Gulf of Mexico. Practically all of the stream water in South America drains into the Atlantic. Do you see why?

33. The Riparian Zone This is the highly vegetated border along most stream channels, providing the transition between aquatic and land environments.

34. Importance of the Riparian Zone Aside from stabilizing a channel (through root systems), the riparian zone acts as a filter for land-derived pollutants, and provides food, shade and habitat for wildlife around the stream. Conservationists call these riparian buffers.

35. In natural streams, floods occur periodically when seasonal discharges grow extremely large and waters exceed channel banks. The first thing that happens after water goes over bank is the velocity drops, and sediment is deposited. This makes natural levees along channels. Floods

36. Flood Recurrence Intervals: Not Prediction A graph such as this can be constructed for any stream where data exist. The more long-term data available, the closer to being ‘predictive’ these become.

37. Flood Data

38. Floods and Urbanization surface runoff vs. infiltration natural land cover vs. urban area

39. Houston Flooding, 2001 urbanization (#4 city in US) + near sea level + tropical storm season =

40. Prior to mass urbanization of floodplains, natural floods were not a problem to society. We now attempt to control floods on most large stream systems by regulating flow with dams and similar structures. Nonetheless, natural flooding is important: Floods: Good or Bad? • sediment deposits on floodplains contribute to the formation of nutrient-rich soils • floodplain wetlands are important habitats for plants and animals

41. Some of the images in this presentation come from: Marshak, Earth: Portrait of a Planet (1st ed) Hamblin and Christiansen, Earth’s Dynamic Systems (8th ed) Press and Siever, Understanding Earth (3rd ed) USGS Bulletin 1471 The Chesapeake Bay Program Credits