Surface Water RIVERS AND STREAMS Rivers and streams

Surface Water

RIVERS AND STREAMS • Rivers and streams are major forces for erosion and transport on Earth's surface. They erode billions of tons of material from the land each year and carry that material to lakes and the ocean. Most of the water which runs back to the ocean in the hydrologic cycle flows through the Earth's rivers.

Sediment Transportation Streams transport billions of tons of clastic sediments downstream every year; these clastic sediments are the load carried by the river. There are two parts: • Suspended Load Made up of fine-grained clays and silts and some sand. This sediment is transported inside the water column as the water flows downstream. • Bed Load Made up of coarse clastic material which rolls and slides along the river bed, rather than being suspended in the water column

Sediment Transportation

Sediment Transportation • Streams may carry big particles or small ones; they may carry large amounts of sediment or only a bit. Two terms: – Competence A measure of the stream's ability to transport large clastic particles. Generally, the faster the current, the higher the competence. – Capacity How much sediment (of all sizes) the stream can transport. Generally, higher volumes of water have higher capacity.

Sediment Transportation • A stream high in the mountains may have a very high current, but not much volume. Such a stream can transport large particles, but not much sediment overall; it has high competence, but low capacity.

Sediment Transportation • A major lowland river like the lower stretches of the Mississippi generally has a lower current, but very large volume. This kind of stream has a low competence, but an enormous capacity.

Sediment Transportation • A river's competence and capacity can change greatly along its length due to changes in slope (which change current speed) and changes in the volume of water in the river.

Sediment Transportation • A river's competence and capacity can change greatly along its length due to changes in slope (which change current speed) and changes in the volume of water in the river. Still bigger particles don't float at all, but instead roll along as part of the bed load.

BEDFORMS • As sediment is transported down a river bed, certain bedforms tend to develop. The two major ones are ripples and dunes. • Current Speed Main Bedform slowest flat river bed slow ripples faster dunes still faster dunes with ripples on the dunes fastest flat river bed with lots of saltating

BEDFORMS Linguoid current ripples, Kennetcook River Straight-crested current ripples, Cambrian sandstone, Moraine Lake, Alberta

Straight-crested dunes, also know as sand waves. Kennetcook River estuary, Nova Scotia

Stream Valleys • A generic river valley consists of high land on either side (called uplands), with a flattened valley of variable width in between. Inside this flattened area is a primary channel, which is where the river flows in most conditions. During floods, the river breaks out of this channel into the surrounding flat floodplain

Channels • There a few general kinds of channels, including: • Straight IT JUST GOES STRAIGHT. • Meandering • Braided Types of stream channels:

Meandering River curves side-to-side; the meanders are the bends in the river. How meanders start is not yet fully understood, but they are common features of rivers. Once meanders have developed, they tend to migrate. The current in a river flows most quickly near the outer edge of a meander and most slowly near the inner edge. Since erosion increases as current speed increases and deposition increases as current speed drops, rivers erode material on the outside of meanders and deposit sediment on the inside. Over time, these processes cause the meanders tend to get more and more curved.

Meandering

Braided A network of meandering channels embedded in a broad valley. This type of channel is often found in rivers with a high sediment load flowing through relatively flat terrain; a prime environment for them is along the outwash plain of glaciers.

Braided

Floodplains The floodplain is the flattish part of the river valley outside the main channel; generally the floodplains are at or slightly below the level of the top of the main river channel. Floodplains are formed by sediment deposited during floods and during migration of meanders.

Floodplains • Many of the world's cities are built on floodplains, which is generally a pretty dumb idea -- after all, if you build a city on a river's floodplain, sooner or later, it's pretty likely to be flooded.

Floodplains So there must be good reasons why cities are built on floodplains. Some good reasons are • Floodplains often have very fertile soil • Rivers can be used for transport, and you want to be as near to the river as possible • Rivers are ready sources of drinking water and water for industries • People may like to live near the water for recreation or simply walks along the riverbanks.

Discharge • What is a flood, anyway? It is, of course, too much water flowing down a river and causing the river to overflow. But how do you measure the volume of water flowing downstream?

Discharge • The volume of water flowing past a fixed point along a river in a given period of time is called the discharge. Generally, it is measured in units of cubic meters per second or cubic feet per second. An equation for discharge is: • Discharge = (cross-sectional area) X (flow speed)

Discharge • Cross-sectional area is the river's width multiplied by its depth. As you can see from this equation, if either the cross-sectional area goes up (because the river gets deeper and/or wider) or the flow speed increases, discharge will also increase. Discharge usually goes up because both go up together. Discharge naturally tends to increase as you go downstream in a river, because secondary tributary streams add their water to the main river.

Floods • Floods are really just times when a river has such unusually high discharge that the river overflows its banks. Floods, of course, come in different sizes. Floods with high discharge (greater severity) occur less frequently than do floods with lower discharge (lower severity). The average time interval between floods of a given severity on a given river is called the recurrence interval.

Floods • A flood with a recurrence interval of N years on a given river is called an N-year flood, where N is some number like 5 years or 1000 years. Downtown Fairbanks during the August 12 -18, 1967, flooding of the Chena and Tanana rivers. The flooding caused six deaths, $85 million in damage, and the evacuation of 12, 000 people. Photograph, UAF archives.

DRAINAGE BASIN • Rivers drain water away from one location and transport it to others. The region which is drained by a given stream system is called the drainage basin of that stream system. Drainage basins may be small or may cover hundreds of thousands of square kilometers.

Drainage basins • Drainage basins are separated from each other by divides. Divides are high ground, like a mountain range or a single hill, where rain which falls on one side flows into one stream system and rain which falls on the other side flows into another stream system. The primary divide for the North American continent is the Rocky Mountains, which divide rain which flows to the Pacific Ocean from rain which flows to the Atlantic Ocean.

Drainage basins • The networks of rivers and tributary streams which make up drainages have several characteristic forms called drainage patterns.

Deltas • The end of a river system comes when the river runs into a large body of water like a big lake or the ocean. When this happens, the river deposits sediment load in a structure called a delta.

Deltas • Not all the sediment gets deposited in the same place. There are three general divisions in the deposition at deltas: • Topset beds. • Foreset beds • Bottomset beds

Deltas • Topset beds Formed from coarse to medium sands which are dropped first as the river slows. These sediments are deposited at the top of the stack of sediments in the delta - hence the name.

Deltas • Foreset beds Formed from fine sands and silt deposited along the front of the delta. These beds slope from near the topset beds to well down into the ocean or lake. The material making up the bed gets finer as you go farther from the mouth of the river.

Deltas • Bottomset beds Formed from fine silt/clay deposited into the deep water in the outermost reaches of the delta. These beds are the end of the delta and grade into the seabottom sediments.

Deltas • Deltas are dynamic environments, involving currents from the river and the ocean, ocean waves, tides, and so on. The sediments carried by the river may be washed away fairly rapidly, keeping the delta small. They may also build up a series of sand bars and beaches ``downwind'' from the delta. Also, deltas evolve over time and may be made up of several palaeodeltas, or previous deltas which have been abandoned. The Mississippi River has occupied and abandoned several deltas over the past 10, 000 years.

Lecture Summary • Here's what I want you to know about, based on these notes, lecture, your notes, and the reading: • Rivers and streams are major erosional forces and transport large amounts of sediment and water to the oceans each year • Load of a river: suspended and bed load • How a river transports sediments • General kinds of bedforms (ripples and dunes). • Generic parts of a river valley • Why meanders migrate once they form • Discharge, floods, and floodplains • Drainage basins, divides, and drainage patterns • Deltas: how they form and what general kinds of sedimentary structures (foreset, topset, bottomset beds, etc. ) form in these environments

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