- How do streams shape the land?
- Stream channels and floodplains
- Braided and meandering streams
- Lakes and waterfalls
- Mature and immature streams
- Stream terraces
- Alluvial fans
- How are shorelines shaped by waves?
- What are the results of glacial erosion and deposition?
- What are the deposits created by wind?
- Web Links
- Glossary Terms
Forces within the earth move its tectonic plates to build mountain ranges and volcanoes. Forces at the earth's surface, which get their energy from the sun, work to wear the mountains down. The sun's energy causes rain and snow to fall and wind to blow, creating the agents that wear away the mountains. Water, ice, and wind are the agents that bit-by-bit wear away and remove the rocks of the mountains, a process called erosion.
Sediments--bits and pieces of the mountains--are transported and deposited into new landforms by the same forces that erode them. Erosion and deposition each create a distinct set of geological features. Erosional features are those left after removal of sediment--valleys, for example. Depositional features, such as river deltas, are built from sediments that are carried from their eroding source and dropped.
On earth, most erosion, transport and deposition of rocks and sediments is performed by liquid water-water flowing in streams and water breaking as waves against shores. Erosion by liquid water is also caused by unchannelled sheets of water flowing down slopes and by seepage of water in the ground. We will focus on streams and breaking waves.
The number two agent of erosion and deposition in the Pacific Northwest is glaciers. Wind is a distant number three.
The agents of erosion and deposition work to bring about a balance between the internal forces of the earth, the energy of the sun and the force of earth's gravity.channels, valleys, floodplains, and deltas.
A stream is a system that seeks balance between the forces within the earth that uplift the land, the energy of the sun that heats the earth's surface, and the force of gravity. As a stream develops, it reaches a finely tuned balance among these forces. Any change to the conditions of the stream will change the nature of the system. The conditions that control the balance of a stream system include the height of the source area, the elevation of the mouth of the stream, and the amount of water and sediment entering the stream. The gradient, or slope, of each part of the stream valley is also important. A change in any of these conditions will cause the stream to respond in a way that moves it back toward a balance.
Within valleys and on floodplains, stream channels shift their location frequently and constantly. Floods commonly inundate floodplains, the flat areas that extend from the edges of the stream channel to the steeper sides of the valley. Through time, stream valleys and their floodplains become wider.
Streams that are carrying an inordinately high amount of sediment down a fairly steep gradient tend to be braided, with many stream channels that divide and recombine. Braided streams are high-energy and have dynamic channel systems that are constantly shifting, especially during high water flow. Streams that drain large glaciers tend to be braided because they receive an over-abundance of sediment from the glaciers.
As streams mature, they come closer to establishing a steady balance between sediment supply, erosion, transport, and deposition. More mature streams also have wider floodplains and gentler stream gradients. A mature stream flowing through a low-gradient floodplain will tend to have a meandering channel--a channel that snakes back and forth in a winding pattern. The channels of meandering streams are not permanently located. The channel will shift its location in the floodplain. Changes in the stream path can happen gradually by erosion and re-deposition of sediments alongside the channel. Or the stream can shift its path abruptly during floods when the stream breaks out of its banks and establishes a new path connecting different parts of its meanders.
As streams mature, along with widening their valleys and floodplains, they fill in lakes with sediments and erode away waterfalls. Lakes and waterfalls are temporary geologic features. They signal that a stream is not yet mature. If an area has lots of lakes and waterfalls, it indicates that the area has undergone a lot of recent geologic activity, which has changed the landscape faster than the streams could keep up with the changes. The recent geologic activity could be volcanic eruptions, uplift of a mountain range, or glaciation. Over time, the streams will work to smooth out and eventually erase many of the effects of volcanism, mountain building, and glaciation. This process can take a long time.
The Pacific Northwest is such a geologically active region that, in general, the streams in the area, including the large rivers, are not mature. They have steep valley walls, narrow floodplains, deep gorges, rapids, and waterfalls. The Columbia River is an example.
The lower parts of some rivers in the region, near where they empty into their base levels (the ocean, a lake, or a larger river), are fairly mature, with wide floodplains, gentle gradients, and meandering stream channels. The lower reaches of some rivers on Puget Sound such as the Snohomish and Skagit Rivers are examples of moderately mature stretches of rivers.
A common geologic occurrence is for the elevation difference from the beginning of a stream to its end to change; in other words, for the overall stream gradient to change. Uplift of the mountains that the streams drain, or lowering of the base level where the streams empty, or a combination of the two can cause an increase in the stream gradient. If the stream gradient undergoes a significant increase, this gives the stream more power. The stream responds by cutting down into the floodplain it has created for itself and establishing a new floodplain at a lower level. Remnants of the old floodplain remain as stream terraces, flat benches along the sides of the valley.
Many streams and rivers in the Pacific Northwest have terraces along the sides of the valleys. Another factor that contributes to the formation of stream terraces is a reduction in the supply of water and sediment to a stream, which leaves a smaller stream that forms a smaller floodplain for itself inside the old, larger floodplain. This was a common occurrence as glaciers finished retreating and melting away at the end of the Pleistocene epoch, and in extreme cases the remaining stream is called an underfit stream. The retreat of glaciers and continued uplift of the mountains has steepened stream gradients. Furthermore, erosion has lowered the base levels of some streams. The streams have responded by digging down and establishing new floodplains at lower levels, leaving remnants of the old flood plains stranded along the sides of their valleys. The result is that stream terraces are a common sight in the Northwest.
Where a stream empties into a large body of water, the stream's current slows down and most of the sediments that were being transported by the current are dropped. These sediments dropped at the river mouth accumulate and gradually build up to form a delta, a deposit of alluvium built into a larger body of water at the mouth of a stream. If the stream empties into a body of water that has strong enough currents or tides a delta may not form because the high water energy in the larger body of water will remove the river sediments before a delta can build up. The Columbia River does not have a delta because the steep slope of the ocean floor off its mouth, along with the wind, tides and ocean current, are too energetic for the sediments to accumulate into a delta. Many smaller streams and rivers in the Northwest, however, do have deltas.
When a stream descends a steep mountain slope and then enters a relatively flat valley, it deposits a cone-shaped landform called an alluvial fan. There are many alluvial fans in the Pacific Northwest. Streams often build up and maintain alluvial fans through flooding and debris flow. Excess water and sediment can cause the stream to flow beyond its usual channel and carry sediment to outlying parts of the alluvial fan. This represents a hazard for people or structures occupying the alluvial fan.
Waves along shorelines erode, transport, and deposit sediments. These processes lead to the sand that resides in a single part of a beach this year being replaced by new sand the following year. Along many shorelines, the wind and waves tend to come from a prevailing direction, leading to steady longshore currents (currents flowing parallel to the shoreline, driven by the wind and waves), and littoral drift (the sediments transported and deposited along the shore by waves and currents).
Steep, rugged, rocky coasts are a sign that geological activity has uplifted or exposed the shore faster than the waves have kept up. Over time, waves will even out a shoreline, filling in the mouths of bays, and eroding back the - promontories. Low coasts with long, straight, sandy beaches are coasts where wave action has taken over.
Erosional features created by waves include sea cliffs, sea stacks, and wave-cut benches or terraces. Depositional landforms created by waves include beaches (which consist of loose sediment), sand spits, and bay-mouth bars.
Just as rivers are a finely tuned balance between erosion, transport, and deposition of sediments, so are shorelines. A change in any of the factors involved in the supply and movement of sediment supply along a shoreline will change the shoreline. If rivers that supply sediments are dammed, or if eroding sandy cliffs are sealed in by sea walls, then the sediment supply in a particular area is reduced. With reduced sediment supply the balance tips toward erosion, and the beach in that stretch of shoreline is likely to be carried away.
Glaciers are bodies of flowing ice. The ice may slide along its base in some places, but for the most part moves by plastic flow. Because glaciers flow, have large volumes and masses, and contain large amounts of energy, they are powerful and effective agents of erosion, transport, and deposition.
Landforms sculpted by glacial erosion include:
- horns--sharp, pyramid-shaped mountain peaks eroded into by glaciers on several sides
- arêtes--sawtooth-jagged ridges eroded into by glaciers on both sides
- cirques--steep-walled amphitheaters at the upper ends of alpine glacial valleys
- glacial troughs--the U-shaped valleys that form under elongate, confined glaciers
- hanging valleys--troughs eroded by smaller tributary glaciers that join up with a deeper U-shaped valley eroded by the main trunk glacier; a steep cliff usually marks the location where a hanging valley joins the main glacial trough
Glacial drift refers to all the types of sediment that a glacier causes to be deposited. This includes:
- glacial till--unsorted, unbedded mixtures of clay, silt, sand, gravel, cobble and boulders; some of the boulders are partly flattened and scraped where they were dragged at the base of the glacier
- glacial outwash--bedded sand and gravel deposited by high-energy meltwater flowing away from the glacier
- varved silt and clay--varves are layers of quartz-rich silt alternating with layers of clay, which accumulate in glacier-related lakes. Such lakes freeze each winter and melt each summer. While the lake is frozen and not receiving melt water sediment, clay particles, which have been suspended, settle to the bottom, forming a clay layer. The silt layer forms during the summer melt, when the lake is receiving lots of runoff.
- glacial erratics--large boulders that were transported by a glacier and left sitting on the ground when the glacier retreated
Glaciers are also effective at building up new landforms from deposits of glacial drift. Depositional glacial landforms include:
- terminal moraines--deposits of glacial till that build up at the terminus or snout of a glacier
- lateral moraines--deposits of glacial till that build up along the sides of a glacier. Lateral moraines usually appear as steep ridges of loose sediment.
- ground moraines--deposits of glacial till that are formed beneath a glacier. Ground moraines tend to be thin and compacted deposits of till.
- kames--hills or ridges of water-laid sediment with one or more steep sides where they were in contact with a glacier
- kettles--holes left where large pieces of glacial ice were stranded by a retreating glacier. Kettles are commonly filled with water, forming kettle lakes.
- eskers--meandering ridges of sediment deposited by meltwater that flows through an ice tunnel at the base of a glacier
- outwash plains--broad, flat areas filled with glacial outwash
In geology, the erosion, transport, and deposition of sediments by wind are called eolian processes. Erosion by the wind is usually minor compared to erosion by water. One exception is broad, flat, enclosed desert basins, which receive sediment from ephemeral, evaporating streams. Wind may gradually remove the finer sediment grains from such basins.
Sand dunes and dune fields require an upwind supply of sand, regular winds, and low, flat terrain that is not being eroded quickly by streams. Loess requires a supply of unusually large amounts of fine-grained sediment and high winds.
During the Pleistocene epoch, when ice sheets covered much of the ground in the Northern Hemisphere, the contrast between the cold ice and the somewhat warmer ground to the south created katabatic winds, strong winds driven by the temperature differences between ice and land. These winds picked up sand and silt from the glacial sediments that had accumulated in large volumes around the margins of the ice sheets, and from other sediment sources in the area. This led to widespread deposits of loess in some areas south of the ice sheets, and to local formation of dune fields.
Loess forms rolling hills of fine-grained sediment that makes fertile soil. Some of the most productive wheat-growing land in the world is located on loess.
To read more about how glaciers work to create their erosional and depositional features and to see illustrations, go the National Snow and Ice Data Center at https://nsidc.org/cryosphere/glaciers and read "All About Glaciers."
Glossary terms that appear on this page: stream channel; stream valley; floodplain; delta; gradient; braided stream; mature stream; alluvium; meandering stream; immature stream; base level; stream terrace; underfit stream; delta; alluvial fan; debris flow; longshore current; littoral drift; sea stack; sand spit; bay-mouth bar; plastic; arête; cirque; hanging valley; glacial trough; terminal moraine; lateral moraine; ground moraine; kame; kettle; esker; outwash plain; glacial drift; glacial till; glacial outwash; varve; glacial erratic; eolian; sand dune; loess; ice sheet
© 2001 Ralph L. Dawes, Ph.D. and Cheryl D. Dawes