Base level: Facts, Discussion Forum, and Encyclopedia Article

Base level

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	Topics Base level Topic Home The base level of a riverRiver A river is a large natural waterway.... or streamStream Overview A stream, brook, beck, burn or creek, is a body of water with a detectable current, confined within ... is the lowest point to which it can flow, often referred to as the 'mouth' of the river. For large rivers, sea levelSea level Summary Mean sea level is the average height of the sea, with reference to a suitable reference surface.... is usually the base level, but a large river or lakeLake A lake is a body of water or other liquid of considerable size surrounded by land.... 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 RiverJordan River The Jordan River is a river in Southwest Asia flowing through the Great Rift Valley into the Dead Sea.... , for which the base level is the Dead SeaDead Sea The Dead Sea is both the lowest point in Eurasia at 418 metres below sea level and falling, and the deepest hypersaline la... , 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 topographyKarst topography Karst topography is a three-dimensional landscape shaped by the dissolution of a soluble layer or layers of bedrock, usually... , a network of sinkholeSinkhole Sinkholes, also known as sinks, shakeholes or dolina, and cenotes, are a feature of landscapes that ... s and caverns that can develop as slightly acidic groundwater enlarges joints (by solutionSolution In chemistry, a solution is a homogeneous mixture composed of one or more substances, known as solutes, dissolved i... ) in limestoneLimestone Limestone is a sedimentary rock composed largely of the mineral calcite .... 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 gradientStream gradient Summary Stream gradient is the ratio of drop in a stream per unit distance, usually expressed as feet per mile or meters per kilomet... ), 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 meanderMeander A meander is a bend in a river, also known as an oxbow loop.... 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 RiverNiagara River The Niagara River flows to the north from Lake Erie to Lake Ontario.... . Eventually this will be cut through as Niagara FallsNiagara Falls Overview Niagara Falls is a set of massive waterfalls located on the Niagara River in eastern North America, on the border between th... recedes, and Lake ErieFacts About Lake Erie Lake Erie is the eleventh largest lake on Earth and, of the five Great Lakes of North America, it is the fourth largest by ... will be drained to near the lower base level of Lake OntarioLake Ontario Lake Ontario , bounded on the north by Ontario and on the south by Ontario's Niagara Peninsula and by New York State, is one... . Movements of land can greatly affect base levels. As an illustration, the uplift of the Colorado PlateauColorado Plateau The Colorado Plateau, also called the Colorado Plateaus Province, is a physiographic region of the Intermontane Platea... , combined with the opening of the Gulf of CaliforniaGulf 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 RiverColorado River Summary The Colorado River is a river in the southwestern United States and northwestern Mexico, approximately 1,450 mi long, draini... and led to the (geologically) rapid erosion that resulted in the Grand CanyonGrand Canyon The Grand Canyon is a very colorful, steep-sided gorge, carved by the Colorado River, in the U.S.... . The many meanders of the Kentucky RiverKentucky River The Kentucky River is a tributary of the Ohio River, 259 mi long, in the U.S.... show that it was a mature stream with little relief. Uplift of the central KentuckyKentucky The Commonwealth of Kentucky is a U.S.... 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 LakesFinger Lakes The Finger Lakes are glacially formed lakes in upstate New York, mainly linear in shape, each lake oriented on a north-south... of New YorkNew York New York is a state in the northeastern United States.... , Ice ageFacts About Ice age An ice age is a period of long-term downturn in the temperature of Earth's climate, resulting in an expansion of the contine... glaciationGlacier A glacier is a large, long-lasting river of ice that is formed on land and moves in response to gravity.... 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 AmericaNorth America North America is a continent in the Earth's northern hemisphere and almost fully in the western hemisphere.... , river valleys extend out onto the continental shelfContinental shelf The continental shelf is the extended perimeter of each continent, which is covered during interglacial periods such as the ... , 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 bays, indicating that sea level has risen in recent geologic time. This is referred to as a drowned coastline.