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. 16.6). Figure 16.7 ( = 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).
