Stream Transport: From Mountains to Oceans - Principles of Geology | GEOL 101, Study notes of Geology

Download Stream Transport: From Mountains to Oceans - Principles of Geology | GEOL 101 and more Study notes Geology in PDF only on Docsity! Chapter 18 – Stream Transport: From Mountains to Oceans A stream is any flowing body of water, large or small. A river is the name for the major branches of a large stream system. Streams are used as commercial waterways, fresh water resources, they provide irrigation for agriculture and natural hazards as well (floods). (Definition) The floodplain is the part of the valley that is flooded when a stream channel overflows. They may be narrow or absent in steep valleys. Floodplain deposits are mainly silt and clay. Channel deposits are mainly sand and gravel. (Figure 18.1) Low-velocity, low-sediment streams that flow on floodplains tend to meander. They shift from side to side in a snaking (serpentine) motion. The current is faster at outside banks, which are eroded, and sediments get deposited in inside bands where the current is slower, forming point bars. As the erosion and deposition process continues, the bends grow closer as a result of growing bigger and point bars get bigger also. During a major flood, when velocity and water volume increase, the river takes a new, shorter course, cutting across the loop. The abandoned loop remains as an oxbow lake. These meandering streams have a single channel with a sinuous pattern. (Key Figure 18.3) High-velocity, high-sediment streams flow over nearly flat, easily eroded terrain (e.g. at the mouths of canyons or the terminal ends of melting glaciers), the fast-moving, sediment-laden water does not form oxbow bends but cuts across the soft sediments at the edges of existing channels, creating shallow, crisscrossed braided channels. Braided streams have an interlacing network of channels because sediment supply is too great for the stream to carry it all. (Key Figure 18.3) Sediments deposited by flooding of a stream channel build of low levees. When flooding occurs water spreads over the floodplain. As water leaves the channel, it rapidly loses velocity and drops its sediment along the immediate borders of the channel. Successive floods build up natural levees that confine the stream to its channel between floods, even when the water is high. Storm surges in coastal areas can overwhelm levees. (Figure 18.4) A drainage basin is an area of land that funnels all water that falls on it into a network of streams. Boundaries of the basin are called divides. (Figure 18.6) Waterfalls retreat upstream as stream beds erode. Falling water undercuts the base of the cliff. (Figure 18.12) Laminar flow almost never exits. This is straight or gently curved streamlines that run parallel without mixing or crossing. Turbulent flows have crossing, mixing streamlines that form swirls and eddies. Turbulence depends of flow velocity (rate of movement), geometry (primarily depth of channel), and viscosity (measures a fluid’s resistance to flow). (Figure 18.13) The hydrosphere and lithosphere interact to transport sediments in streams. Current flowing over a bed of gravel, sand, silt and clay carries a suspended load of finer particles and a bed load of material sliding and rolling along the bottom. As current velocity increases, the suspended load grows and the increased shear of the bed generates an increase in the bed load. Particles move by saltation, jumping along the bed. At a given current velocity, smaller grains jump higher and travel farther than larger grains. (Figure 18.14) A typical large marine delta, many kilometers in extent, in which the fine-grained foreset beds are deposited at a very low angle (4-5 deg or less) is shown in this figure. Sandbars form and the mouths of the distributaries, where the currents’ velocity suddenly decreases. The delta builds forward by the advance of the bar and the topset, foreset, and bottomset beds. Between distributary channels, shallow bays fill with fine-grained sediment and become salt marshes. This general structure is found on the Mississippi delta. Some currents follow the slope and hug the bottom. As they decelerate, they drop