Base level
Encyclopedia
The base level of a river
or stream
is the lowest point to which it can flow, often referred to as the 'mouth' of the river. For large rivers, sea level
is usually the base level, but a large river or lake
is likewise the base level for tributary streams. All rivers and streams erode toward sea level, which is also known as the "ultimate base level." If a river is dammed, a new base level (the level of the reservoir) replaces the ultimate base level. As a result, the stream’s base level is raised. Consequently, this reduces the stream’s velocity, leads to deposition, and a reduction of the gradient upstream from the reservoir. A rather rare exception can be seen in the Jordan River, for which the base level is the Dead Sea
, 417 m below modern sea level.
Base level is also significant for subsurface drainage. A low base level is a prerequisite for the formation of karst topography
, a network of sinkhole
s and caverns that can develop as slightly acidic groundwater enlarges joints (by solution
) in limestone
rock. Often this network of underground drainage feeds back to surface drainage along the edges of larger rivers, which are the effective base level.
When the source of a stream is very high relative to its base level (high stream gradient
), erosion proceeds rapidly due to the energy of the rapidly moving water and the topography becomes rugged, and it is considered a young stream (geologically speaking). When erosion has acted for a long geologic time, wearing down the high points and making a small difference between the source and the base level of a stream (low stream gradient), then the stream is called mature. Mature stream valleys have gentle slopes, rounded higher points and meander
ing courses.
A very hard layer of rock can form a temporary base level, until it is cut through. For example, Lockport dolomite has created a temporary base level for the Niagara River
. Eventually this will be cut through as Niagara Falls
recedes, and Lake Erie
will be drained to near the lower base level of Lake Ontario
.
Movements of land can greatly affect base levels. As an illustration, the uplift of the Colorado Plateau
, combined with the opening of the Gulf of California
to the sea, established sea level as the base level for the Colorado River
and led to the (geologically) rapid erosion that resulted in the Grand Canyon
.
The many meanders of the Kentucky River
show that it was a mature stream with little relief. Uplift of the central Kentucky
area rejuvenated the stream, causing it to cut a deep canyon, but preserving the meanders that indicate the stream existed prior to the uplift.
Other geologic processes can affect base levels. In the Finger Lakes
of New York
, Ice age
glaciation
greatly deepened the lake valleys and rejuvenated tributary streams. These streams have deep ravines and often waterfalls where hard rock layers slow erosion; they are said to be hanging valleys.
Sea level can also change (primarily due to the formation or melting of continental ice sheets), and raise or lower the final base level for coastal rivers. Along the east coast of North America
, river valleys extend out onto the continental shelf
, indicating a time when erosion was possible due to the lower base level of a lower sea level in the past. Today many of these same rivers end in bay
s, indicating that sea level has risen in recent geologic time. This is referred to as a drowned coastline.
River
A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...
or stream
Stream
A stream is a body of water with a current, confined within a bed and stream banks. Depending on its locale or certain characteristics, a stream may be referred to as a branch, brook, beck, burn, creek, "crick", gill , kill, lick, rill, river, syke, bayou, rivulet, streamage, wash, run or...
is the lowest point to which it can flow, often referred to as the 'mouth' of the river. For large rivers, sea level
Sea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...
is usually the base level, but a large river or lake
Lake
A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...
is likewise the base level for tributary streams. All rivers and streams erode toward sea level, which is also known as the "ultimate base level." If a river is dammed, a new base level (the level of the reservoir) replaces the ultimate base level. As a result, the stream’s base level is raised. Consequently, this reduces the stream’s velocity, leads to deposition, and a reduction of the gradient upstream from the reservoir. A rather rare exception can be seen in the Jordan River, for which the base level is the Dead Sea
Dead Sea
The Dead Sea , also called the Salt Sea, is a salt lake bordering Jordan to the east and Israel and the West Bank to the west. Its surface and shores are below sea level, the lowest elevation on the Earth's surface. The Dead Sea is deep, the deepest hypersaline lake in the world...
, 417 m below modern sea level.
Base level is also significant for subsurface drainage. A low base level is a prerequisite for the formation of karst topography
Karst topography
Karst topography is a geologic formation shaped by the dissolution of a layer or layers of soluble bedrock, usually carbonate rock such as limestone or dolomite, but has also been documented for weathering resistant rocks like quartzite given the right conditions.Due to subterranean drainage, there...
, a network of sinkhole
Sinkhole
A sinkhole, also known as a sink, shake hole, swallow hole, swallet, doline or cenote, is a natural depression or hole in the Earth's surface caused by karst processes — the chemical dissolution of carbonate rocks or suffosion processes for example in sandstone...
s and caverns that can develop as slightly acidic groundwater enlarges joints (by solution
Solution
In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
) in limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
rock. Often this network of underground drainage feeds back to surface drainage along the edges of larger rivers, which are the effective base level.
When the source of a stream is very high relative to its base level (high stream gradient
Stream gradient
Stream gradient is the grade measured by the ratio of drop in a stream per unit distance, usually expressed as feet per mile or metres per kilometre.-Hydrology and geology:A high gradient indicates a steep slope and rapid flow of water Stream gradient is the grade (slope) measured by the ratio of...
), erosion proceeds rapidly due to the energy of the rapidly moving water and the topography becomes rugged, and it is considered a young stream (geologically speaking). When erosion has acted for a long geologic time, wearing down the high points and making a small difference between the source and the base level of a stream (low stream gradient), then the stream is called mature. Mature stream valleys have gentle slopes, rounded higher points and meander
Meander
A meander in general is a bend in a sinuous watercourse. A meander is formed when the moving water in a stream erodes the outer banks and widens its valley. A stream of any volume may assume a meandering course, alternately eroding sediments from the outside of a bend and depositing them on the...
ing courses.
A very hard layer of rock can form a temporary base level, until it is cut through. For example, Lockport dolomite has created a temporary base level for the Niagara River
Niagara River
The Niagara River flows north from Lake Erie to Lake Ontario. It forms part of the border between the Province of Ontario in Canada and New York State in the United States. There are differing theories as to the origin of the name of the river...
. Eventually this will be cut through as Niagara Falls
Niagara Falls
The Niagara Falls, located on the Niagara River draining Lake Erie into Lake Ontario, is the collective name for the Horseshoe Falls and the adjacent American Falls along with the comparatively small Bridal Veil Falls, which combined form the highest flow rate of any waterfalls in the world and has...
recedes, and Lake Erie
Lake Erie
Lake Erie is the fourth largest lake of the five Great Lakes in North America, and the tenth largest globally. It is the southernmost, shallowest, and smallest by volume of the Great Lakes and therefore also has the shortest average water residence time. It is bounded on the north by the...
will be drained to near the lower base level of Lake Ontario
Lake Ontario
Lake Ontario is one of the five Great Lakes of North America. It is bounded on the north and southwest by the Canadian province of Ontario, and on the south by the American state of New York. Ontario, Canada's most populous province, was named for the lake. In the Wyandot language, ontarío means...
.
Movements of land can greatly affect base levels. As an illustration, the uplift of the Colorado Plateau
Colorado Plateau
The Colorado Plateau, also called the Colorado Plateau Province, is a physiographic region of the Intermontane Plateaus, roughly centered on the Four Corners region of the southwestern United States. The province covers an area of 337,000 km2 within western Colorado, northwestern New Mexico,...
, combined with the opening of the Gulf of California
Gulf of California
The Gulf of California is a body of water that separates the Baja California Peninsula from the Mexican mainland...
to the sea, established sea level as the base level for the Colorado River
Colorado River
The Colorado River , is a river in the Southwestern United States and northwestern Mexico, approximately long, draining a part of the arid regions on the western slope of the Rocky Mountains. The watershed of the Colorado River covers in parts of seven U.S. states and two Mexican states...
and led to the (geologically) rapid erosion that resulted in the Grand Canyon
Grand Canyon
The Grand Canyon is a steep-sided canyon carved by the Colorado River in the United States in the state of Arizona. It is largely contained within the Grand Canyon National Park, the 15th national park in the United States...
.
The many meanders of the Kentucky River
Kentucky River
The Kentucky River is a tributary of the Ohio River, long, in the U.S. state of Kentucky. The river and its tributaries drain much of the central region of the state, with its upper course passing through the coal-mining regions of the Cumberland Mountains, and its lower course passing through the...
show that it was a mature stream with little relief. Uplift of the central Kentucky
Kentucky
The Commonwealth of Kentucky is a state located in the East Central United States of America. As classified by the United States Census Bureau, Kentucky is a Southern state, more specifically in the East South Central region. Kentucky is one of four U.S. states constituted as a commonwealth...
area rejuvenated the stream, causing it to cut a deep canyon, but preserving the meanders that indicate the stream existed prior to the uplift.
Other geologic processes can affect base levels. In the Finger Lakes
Finger Lakes
The Finger Lakes are a pattern of lakes in the west-central section of Upstate New York in the United States. They are a popular tourist destination. The lakes are long and thin , each oriented roughly on a north-south axis. The two longest, Cayuga Lake and Seneca Lake, are among the deepest in...
of New York
New York
New York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...
, Ice age
Ice age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...
glaciation
Glacier
A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...
greatly deepened the lake valleys and rejuvenated tributary streams. These streams have deep ravines and often waterfalls where hard rock layers slow erosion; they are said to be hanging valleys.
Sea level can also change (primarily due to the formation or melting of continental ice sheets), and raise or lower the final base level for coastal rivers. Along the east coast of North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
, river valleys extend out onto the continental shelf
Continental shelf
The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...
, indicating a time when erosion was possible due to the lower base level of a lower sea level in the past. Today many of these same rivers end in bay
Bay
A bay is an area of water mostly surrounded by land. Bays generally have calmer waters than the surrounding sea, due to the surrounding land blocking some waves and often reducing winds. Bays also exist as an inlet in a lake or pond. A large bay may be called a gulf, a sea, a sound, or a bight...
s, indicating that sea level has risen in recent geologic time. This is referred to as a drowned coastline.