Rivers: Transport to the Oceans
Streams play the major role in shaping the face of the continental landscape: 1) Erode Mountains, 2) Transport sediments to oceans, 3) Deposit billions of tons of sediments in bars and floodplains and 4) Deposit even more sediment in deltas.
How Stream Water Flows
Laminar – straight or gently curving streamlines. No mixing or crossing.
Turbulent – streamlines mixing, crossing, and forming swirls and eddies. Most streams are turbulent flow.
Stream Loads and Sediment Movement - Turbulent flows move particles by suspension, saltation, or bed load. For sand and larger particles, stream velocity determines whether a particle is deposited, transported, or eroded. Higher velocities favor erosion
Suspension – small particles that remain up in the water column
Saltation – larger particles move by short jumps. Dunes and ripples - form by saltation of sand grains. They migrate downstream
Bed load – large particles move by rolling or sliding
Competence – maximum size particles a stream can carry
Capacity – amount of material (sediment) a stream can carry.
Running Water Erodes Solid Rock by - Abrasion – 1) sandblasting by sand and gravel; 2) Chemical and Physical Weathering; and 3) Undercutting Action by Currents at base of waterfalls
Channels and Floodplains
Meanders – channels follows curves and bends. Meanders erode on outside of curves (highest stream velocities) and deposit on inside of curves (lowest stream velocities) creating point bars. Meanders move side to side and also migrate downstream over time. Cut off meanders form oxbow lakes (e.g., False River).
Braided Streams – many channels that split apart and rejoin.
Natural Levees – coarse sediment along a narrow strip at the edge of the channel.
Floodplains – slit and clay plus organics deposited when flood waters evaporate.
Streams Change with Time and Distance
Discharge - volume of water that passes a given point in a given time (cubic meters per second). Discharge of most streams increases downstream because of additional water from smaller streams (tributaries) that flow into larger streams.
Floods - Discharge increases so much that water is no longer confined to the channel. Floods occur at regular intervals. Small floods occur frequently. Large floods occur infrequently
Longitudinal Profile - slope of river from headwaters to mouth is a concave upward profile (steep slope at headwaters to almost level slope at mouth). Streams are in a dynamic equilibrium between erosion and sedimentation. Longitudinal Profile is controlled at its lower end by baselevel (the ocean or a lake). Profiles change when baselevel changes. Damming a river creates a new baselevel
Alluvial Fans - at a mountain front, streams spread out, slow down and deposit material. Coarsest material is deposited near the mountain front.
Terraces - Former floodplains that existed at a higher level before uplift, lowering of sea level or an increase in discharge caused the river to erode into its floodplain
Lakes - lakes in Northern U.S. are a consequence of disruption of drainage by the last ice age. They will eventually disappear.
Drainage Networks - Each river has a drainage basin separated by divides (topographic highs). The Mississippi river drains the central U.S. Drainage Patterns - 1) Dendritic - irregular branching pattern; 2) Rectangular - streams follow fractures or joints in bedrock; and 3) Radial - streams run outward in a radial pattern from a central high point.
Deltas: The Mouths of Rivers - As a stream enters the ocean it slows down. The coarsest material, normally sand, is dropped first at the mouth of the river. Finer-grained sands are dropped farther out, followed by silt and clays. Deltas grow outward into the ocean for hundreds or thousands of years and then switch to a shorter path to the sea.
Distributaries - At a delta, the river breaks up into smaller streams.
Most deltas have an internal structure of topset (sand, horizontal), foreset (fine sand and silt, inclined), and bottomset (mud, horizontal) beds.
Flood control dams and artificial levees are reducing the amount of sediments to wetlands increasing coastal erosion.