Learning Objectives

1. Running water (aided by mass wasting) is the most important geologic agent for erosion, transportation, and deposition of sediment and landscape development on earth.

2. The longitudinal profile of a stream changes from steep to gentle as the stream flows from its headwaters (where valleys are V-shaped) to its mouth (where valleys are surrounded by a flat flood plain). Stream channels usually contain the stream, but unchanneled sheetwash can occur, commonly in deserts.

3. Streams drain drainage basins separated from each other by divides. Drainage patterns reflect rock type and structure. Dendritic drainages form on horizontal, unfractured bedrock. Radial drainages form on high conical mountains. Rectangular drainages form on fractured or jointed bedrock. Trellis drainages form in areas of tilted bedrock of varying resistance to erosion.

4. Stream erosion and deposition are controlled by velocity and discharge. Velocity is the distance water travels per unit of time. Maximum velocity is near the middle of the water column and is displaced to the outside of its curves (Fig. 10.6). Figure 10.7 ( = Hjulstrom's Diagram but not labeled as such) illustrates that as velocity increases (for example during a flood), erosion and transportation of larger grain sizes is accomplished. The point is also made that more velocity is required to erode silt and clay than sand. Gradient (high vs low), channel shape (narrow vs wide). roughness (smooth vs rough), and discharge (increased volume of water) influence velocity.

5. Stream erosion involves hydraulic action (ability to pick up and move sediments), solution, and abrasion (grinding of stream bed by coarse sediment load, resulting in potholes).

6. Stream transportation of sand and gravel is accomplished as bed load (movement by traction that maintains contact with the stream bed or saltation that involves bouncing along stream bed). Silt and clay are transported by suspension in the water. Dissolved load comprises soluble ions. Suspension and solution comprise the bulk of a stream's load.

7. Stream deposition reflects a drop in velocity. Bars and braided streams are formed usually by gravel deposited as velocity falls in streams with high discharge and bed load, and may contain placer deposits. Meandering in the lower reaches of a stream produce points bars in the inside of meander loops and erode the outside of meander loops. Flood plains are formed by a combination of point-bar deposits, fine-grained flood deposits, and channel-fill. Natural levees reflect drop in velocity and deposition along stream channels during flooding.

8. Deltas and alluvial fans reflect drop in velocity as a stream enters a body of water or at the base of mountains respectively. Sediment supply, waves, and tides control the shape of a delta. Bottomset, foreset and topset beds characterize deltas in freshwater lakes. Alluvial fans usually exhibit grading, with coarsest material deposited closest to the mountain front because of a drop in stream velocity where the channel leaves a canyon.

9. Flooding is a natural process caused by heavy rains and snow melt. Recurrence intervals predict the average time separating flood events, particularly 100-year floods. Flood erosion, high water and flood deposits are the undesirable results of flood events. Urbanization enhances flooding by paved areas, storm sewers, and channel constrictions (bridges, docks, buildings). Flash floods are short lived events often caused by thunderstorms. Two catastrophic flash floods struck north-central Colorado in 1976 (Big Thompson River) and 1997 (Spring Creek, Cache la Poudre River). Flooding may be partially controlled by dams, artifical levees, protective walls, and bypasses, but prohibiting building within 100-year flood plains should be encouraged.

10. The Great Flood of 1993 exceeded 100-year discharges for many rivers in the midwest and even the 500-year flood at Hannibal, Missouri.

11. Erosional downcutting forms stream valleys and is limited by base level, either sea level or a local base level, such as a pond or lake. Glaciation may lower base level promoting downcutting in stream valleys, or raise base level promoting deposition.

12. Ungraded streams use downcutting to smooth their gradients. Graded streams exhibit a balance between capacity and load maintained by downcutting and deposition to smooth their gradients. Graded streams typically exhibit downcutting, lateral erosion accompanied by meandering and valley widening, and headward erosion that lengthens its valley, and produce stream piracy.

13. Stream terraces are either rock benches, or"stepped" sediment. They reflect a change from deposition to erosion caused by either regional uplift (which lowers base level and promotes downcutting), or change from dry to wet climate (which increases the erosional capability of the stream).

14. Incised meanders have no flood plain, as typically found in meandering streams, and reflect either lowered base level or simultaneous downcutting and lateral erosion of graded streams.

15. Superposed streams occur when uplift allows a stream to erode through sediment burying mountain ranges (e.g. folded Appalachians).