Oxbow - Oxbow Lake

Formal/Scientific Name: Oxbow Lake

Designations in other languages: Commonly referred to as ‘dead arm’ in other languages. In French it is bras mort .

Identification: “A curved lake formed at a former oxbow where the main stream of the river has cut across the narrow end and no longer flows around the loop of the bend.”

Cause/Method of Creation: The most common type of lake of fluvial [river] origin, oxbow lakes are the product of a series of geomorphological events, a cut-off meander created by a water source that is formed on a sedimentary floodplain located on a very slight gradient. To understand the creation of oxbow lakes, we need to explain the creation of meanders first. 

Step 1

Step 1: A stream or river on a
nearly-level plain would normally flow in a straight line, but the stream path becomes altered as physical obstructions (stones, boulders and bedrock outcroppings) modify the direction of the waters flow, causing eddies, whirlpools and variations in the water-flow rate from one side of the river to the other. This variation in the water speed causes the current to swing back and forth in the stream bed.

Step 2

Step 2: Where the stream flow is strong at the sides, it begins to erode the bank away, carving out a curve or arc.
Simultaneously, on the opposite side of the stream, the water flow is slower, allowing sediments from upstream to settle, filling in one side of the former stream bed and creating a wedge-shaped sediment deposit called a point bar. So while one side of the streambed is eroding sideways and sending sediment downstream, the opposite side is building up with sediment from upstream. This process slowly relocates the stream bed across the flat plain and creates  bends in the river called meanders.

Step 3

Step 3: Once this erosion and deposition process begins it continues to carve the meander until the loops are so pronounced, they almost form a full circle. Before the circle becomes complete the area of land at the “ends” of the meander (where the point bar first formed and then was consequently eroded away) is now a thin bridge, called a meander neck, separating upstream and downstream points on the river.

Step 4

Step 4: The thin meander neck is finally eroded away and the river, flowing straight, through the cut-off, leaves behind the old, curved route. More sediment deposits from upstream quickly separate the stream from the deserted meander and, with it’s moving water source depleted, the meander has become a U-shaped lake or pond, resembling an ox bow. Some oxbow lakes stay filled with water, fed by other water sources, and some become filled with sediment and plant material. These filled oxbows are called abandoned meanders or meander scars.




Step 5: As the stream wanders over the plain, it continuously forms, and abandons, more oxbow lakes. Aerial photographs (see Aerial View below) of very old meandering streams and rivers show the overlapping, crescent-shaped, soil patterns of abandoned meanders, sometimes quite far from the water source, which has migrated across the plain, obliterating the older abandoned meanders formed long ago.

Formation Table: Oxbows are…
   Exogenic forms (on or near the earths surface), created by
      Erosion–via the Transportation Force of fluid (water) and
                    via the Deposition Means of the physical deposit of sediment. 

Oxbow Formation Table

Variations: Meander scars – former oxbow lakes filled with sediment and organic material. Can be identified in aerial photos long after the lake is gone.
Meander caves – usually formed on karst landscapes. 
Incised meander – form in bedrock of horizontal strata, as in San Juan River, Utah. 

Aerial View: One of the rewards of airline travel is the chance to see landforms form a very unique perspective. Below are aerial photos of rivers in the process of creating new oxbow lakes and eroding away old abandoned meanders. 

credit: USGS

Topo Map View: When reading a topographic map, different types of landforms may have a distinctive shape to the contour lines, making it easy to pick out. The topo map at right gives an idea of what an oxbow lake may look like on a topographic map. Also notice the numerous crescent-shaped meander scars marked by brown contour in the north-west corner and the large sand bars of sediment deposits in
the river. 




















Physical Locations: Oxbow lakes may exist wherever the proper conditions occur. Look for oxbow lakes on wide flat plains with a source of flowing water. The Mississippi River and the Amazon River both have numerous oxbow bends, oxbow lakes and meander scars. Melting glacier water, as well as tropical rivers, can both be sources of meandering rivers, therefore oxbow lakes can be found worldwide.

Access difficulty: Depends on the location and accessibility. Since oxbow lakes form on flat plains, this landform may be easier to access then most.

Natural Decay/Destruction: In comparison to many landforms, lakes have short life-spans, as any depression in the earth is destined to be filled in by sediments and deposits from upstream, as well as from organic material growing along it’s shores. A lake may exist for a few hundred years or even a few thousand years but this represents mere moments in geologic time.

Effect on environment: The still, freshwater in an oxbow lake creates a significant aquatic habitat for wetland and marshland. Tadpoles and young frogs, fish, young turtles, some types of snails and certain aquatic plants favor a setting with calmer waters than a moving river can provide.

Oxbow lakes may behave similar to wetlands in that they act as a sponge for retaining water that is dispersed slowly during dry seasons, thereby providing water to plants and animals at a time when other sources have dried up. Oxbow lakes also capture floodwaters during rainy season, slowing the land erosion and property destruction of an overflowing river.

Effect on humans: Having assumed that a rivers path was permanent, civilizations have been built along meandering rivers, only to have their buildings and farms washed away as the river wandered across it’s flat, sediment plain.

Hoodoos - Mushroom Rocks - Pedestal Rocks

Formal/Scientific name: mushroom rocks, perched rocks  and pedestal rocks .

Designations in other languages: zeugen, which means witness or onlooker
in German.

Identification: A mushroom shaped form with a large bulbous blob resting on a thinner supporting column.

Cause/Method of Creation: The supporting column of mushroom and pedestal rocks are caused by etching, differential weathering , and natural sandblasting of sedimentary deposits. A smoothly rounded top (as opposed to a top with rough edges) is created by spheroidal weathering  into a form resembling a mushroom.

Long before the mushroom shapes begin to form, a sedimentary rock bed has been created by deposition, burial and lithification leaving an erosion resistant layer above softer sediment layers. Joints and fractures form in the upper layer, allowing faster erosion of the soft strata below. Abrasion near the ground by windblown sand, cause the lower portion of a rock mass to be undercut leaving the upper portion unaffected. The higher off the ground the wind is, the faster it moves and can sandblast harder, but the less sand it can carry. The greatest erosion will occur at the height of the optimum balance of wind speed and sand grain size (see illustration below). The base may be highly polished as it receives the most etching from fine grains. 

Note: There is considerable confusion in the use of the labels; mushroom, perched, balanced, pedestal, table and chimney rocks, earth pillar, hoodoo, yardang, ruiniform karst, goblin and demoiselle. This cross naming occurs in both common usage and in academic writings.

For example, the name hoodoo is used in Goblin Valley State Park, Utah, to name the landforms described in this blog entry. Elsewhere in Utah, at Bryce Canyon National Park, the name hoodoo is used locally to describe a chimney rock . It seems that the names hoodoo and goblin are often used for a landform that has an anthropomorphic (human) shape regardless of its method of formation.

The landforms known as balanced, table and chimney rocks, earth pillar, yardang, ruiniform karst, and demoiselle are either similar in shape but formed by a different process than a mushroom rock, or are a different shape formed by a similar process. These landforms will be described in future bogs. The name hoodoo is included in this blog for those who are familiar with this label, although the term seems to be used mostly regionally (and ambiguously) in the American Southwest.

Formation Table: Mushroom/pedestal rocks are… 

  Exogenic forms (on or near the earths surface), created by
     Weathering–via the Mechanical Agent of spheroidal Weathering
     and Erosion–via the Transportation Force of wind 

Mushroom Rock formation table

Variations: May have rounded tops or tops with rough edges depending on the substance of the upper sediment layers.

Aerial View: Having an entirely different appearance form the air, the identifying feature of the pedestal on a mushroom rocks isn’t visible and therefor impossible to identify without already knowing the location coordinates. The Google Earth photo below shows a large concentration of mushroom
rocks represented by the small bumps.

credit: Google Earth

Topo Map View: This is the USGS topographic map for Goblin Valley State Park, UT, which is shown in the map section below. There are no contour marks indicating the mushroom rock formations (they would be scattered around the red number ‘10’ near the center of the map). This may be because the contour interval of the map is 40 feet and the ‘goblins’ avg. about 15 feet in height.

credit: USGS

Physical Locations: The coordinates for where mushroom rocks can be found:

38°34′00″N 110°42′36″W – Goblin Valley State Park, Hanksville, Utah, U.S.

38°43’34”N 098°01’47”W – Mushroom Rock State Park, Marquette, Kansas, U.S.

40°12'23"N 002°00'24"W – La Ciudad Encantada, Cuenca, Spain

Access Difficulty: Depends on the surrounding terrain and accessibility.

Natural Decay/Destruction: Eventually the etched middle section of the pedestal, or mushroom stem, becomes too weak to support the heavier cap and topples over, leaving both the de-capped stem and separated bolder to erode by wind-borne sand and other erosional elements.

Effect on Environment: [Referenced sources have provided no information on this topic. As new sources are discovered, further information may be added to this blog.]

Effect on Humans:This landform has no known negative effect on humans, however mankind seems to be sufficiently intrigued by these people-like shapes that they have created nature parks around especially interesting examples (see Physical Locations above). The dramatic mushroom rocks at La Ciudad Encantada were used as background scenery for the movies The Valley of Gwangi and Conan the Barbarian.

Humans attempt to alter and the consequences: [Referenced sources have provided no information on this topic. As new sources are discovered, further information may be added to this blog.]

Web Sites: Links to further information.

Goblin Valley State Park

Mushroom Rock State Park

Pedestal Rock Scenic Area at Ozark-St. Francis National Forests

Introduction

Landforms, Geomorphology & Geography...

...but not the usual. You won't find information about volcanoes and earthquakes here. GeoQuest is about the unique,"weird" and uncommon landforms found in nature.

Landforms are unavoidable. We are all surrounded by landforms. Even a “featureless desert” has features, you just need to know what to look for. The land we live on has mountains, rivers, valleys, plains, shores, and lakes. These large and obvious land shapes are familiar to many people, but there are hundreds of smaller, less common landforms that are unknown to most. We may live near, or drive past, one or more of these rare features every day, but because they have never been described or pointed out, they are invisible to us.

GeoQuest is an index where each blog entry describes one of these unique, “weird”, uncommon earth features. Each blog entry is presented in a “field report” format where the reader can expect to find explanations on; what causes the landform to be created, specific locations where the landform can be found, photographs and diagrams, variations of the feature, how they decay or are destroyed, benefits to the environment or humans, as well as a reference list of books and web sites about the landform.