In geology and geomorphology a base level is the lower limit for an erosion process . The modern term was introduced by John Wesley Powell in 1875. The term was subsequently appropriated by William Morris Davis who used it in his cycle of erosion theory. The "ultimate base level" is the surface that results from projection of the sea level under landmasses. It is to this base level that topography tends to approach due to erosion, eventually forming a peneplain close to the end of a cycle of erosion.
13-515: Castle Mill Stream is a backwater of the River Thames in the west of Oxford , England . It is 5.5 km long. The stream leaves the main course of the River Thames at the south end of Port Meadow , immediately upstream of Medley Footbridge , split by the northern part of Fiddler's Island to the west. It flows between Port Meadow to the north and Cripley Meadow (largely allotments ) to
26-401: A metaphor to physical and social areas that have been bypassed. It may apply to places that have been neglected in economic development, or in the expression a "cultural backwater". When a section of a river is near the coast or another feature that sets its base level , the section influenced by the conditions at its mouth is termed a backwater. If a river flows into a lake or sea, it is
39-404: A base level also influences the position of deltas and river terraces . Together with river discharge and sediment flux the position of the base level influences the gradient , width and bed conditions in rivers. A relative drop in base level can trigger re-adjustments in river profiles including knickpoint migration and abandonment of terraces leaving them "hanging". Base level fall
52-437: A branch of a main river, which lies alongside it and then rejoins it, or to a body of water in a main river, backed up by the sea tide or by an obstruction such as a dam . Manmade restrictions to natural stream flow or temporary natural obstructions such as ice jams , vegetation blockage, or flooding of a lower stream can create backwater. If a river has developed one or more alternative courses in its evolution, one channel
65-420: Is also known to result in progradation of deltas and river sediment at lakes or sea. If the base level falls below the continental shelf , rivers may form a plain of braided rivers until headward erosion penetrates enough inland from the shelfbreak . When base levels are stable or rising rivers may aggrade . Rising base levels may also drown the lower courses of rivers creating rias . This happened in
78-520: Is delayed by resistant rocks. Examples of this include karst regions underlain by insoluble rock. Base levels may be local when large landmasses are far from the sea or disconnected from it, as in the case of endorheic basins . An example of this is the Messinian salinity crisis , in which the Mediterranean Sea dried up making the base level drop more than 1000 m below sea level. The height of
91-523: Is usually designated the main course, and secondary channels may be termed backwaters. The main river course will usually have the fastest stream and will likely be the main navigation route; backwaters may be shallower and flow more slowly, if at all. Some backwaters are rich in mangrove forest . This results in a more diverse environment of scientific interest and worthy of preservation. Backwaters also provide opportunities for leisure activities such as canoeing and fishing. The term has been applied as
104-618: The Thames. The stream then flows under Hythe Bridge , on Hythe Bridge Street , and under a series of bridges: Pacey's Bridge on Park End Street , Quaking Bridge , and Swan Bridge (once known as Castle Bridge), outside the original medieval city walls of Oxford, near Oxford Castle . It continues under Oxpens Road and rejoins the Thames immediately upstream of the Gasworks Bridge . The stream
117-405: The centre of Oxford, and in 1795-96 Daniel Harris built Isis Lock to allow Thames river traffic to access the canal wharves. The stream then fell out of use for navigation. 51°44′47″N 1°15′49″W / 51.7464°N 1.2637°W / 51.7464; -1.2637 Backwater (river) A backwater is a part of a river in which there is little or no current. It can refer to
130-448: The region in which the slope of the river decreases because the lower water flux permitted at the mouth causes the water to back up. Where the river outlet is strongly affected by tides , the cyclic change in base level changes the portion of the river that is a backwater. As a result, fresh and salt water may become mixed to form an estuarine environment. Base level There are also lesser structural base levels where erosion
143-731: The south. It then passes under the Cherwell Valley railway line and turns south, alongside the southern end of the Oxford Canal and the railway tracks, across which is the Castle Mill graduate housing development of the University of Oxford . Further south, the Isis Lock gives access to the Oxford Canal, and the short Sheepwash Channel leads west under the railway tracks to the main stream of
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#1733055898210156-540: The walls. She was clothed in a white dress that camouflaged her against the snow. Matilda crept through the enemy lines and across the Castle Mill Stream to escape to freedom. In the Middle Ages , the stream was used for navigation, at least above Hythe Bridge. During the 16th century, hay, wood, stone, and slate were unloaded at a wharf at Hythe Bridge. When the Oxford Canal was built, it provided an easier route into
169-446: Was altered greatly in Saxon and early Norman times. There was a mill here even before the castle existed. In the winter of 1142, there was a siege at the castle. Empress Matilda (aka Queen Maud, 1102–1167), the daughter and dispossessed heir of Henry I , was there during her power struggle with King Stephen (1096–1154). The queen escaped from the castle after her guards lowered her over
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