page_title: Base level
hierarchical_section_title: Base level
caption_attribution_description: English: Aerial picture of the Ebro river's mouth as it enters into the Mediterranean sea by the Ebro's delta. Català: Fotografia aèria del riu Ebre al seu tram final desembocant a la mar Mediterrània pel delta de l'Ebre Español: Fotografía aérea del río Ebro en su tramo final desembocando en el mar Mediterráneo por el delta del Ebro.
context_page_description: 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 plane 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.
  There are also lesser structural base levels where erosion 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 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.
context_section_description: 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 plane 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. 
  There are also lesser structural base levels where erosion 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 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 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 the Nile during the Zanclean flood when its lower course became, in a relatively short time, a large estuary extending up to 900 km inland from the Mediterranean coast.
  Base level change may be related to the following factors:
  Sea level change
  Tectonic movement
  River capture
  Extensive sedimentation
caption: Aerial picture of the Ebro river as it reaches the Mediterranean Sea by the Ebro Delta
exif: {"Image Make": "Canon", "Image Model": "Canon EOS 5D Mark II", "Image Orientation": "Horizontal (normal)", "Image XResolution": "140", "Image YResolution": "140", "Image ResolutionUnit": "Pixels/Inch", "Image Software": "Adobe Photoshop CS4 Windows", "Image DateTime": "2013:09:27 15:03:01", "Image ExifOffset": "216", "Thumbnail Compression": "JPEG (old-style)", "Thumbnail XResolution": "72", "Thumbnail YResolution": "72", "Thumbnail ResolutionUnit": "Pixels/Inch", "Thumbnail JPEGInterchangeFormat": "774", "Thumbnail JPEGInterchangeFormatLength": "4075", "EXIF ExposureTime": "1/500", "EXIF FNumber": "9", "EXIF ExposureProgram": "Shutter Priority", "EXIF ISOSpeedRatings": "200", "EXIF ExifVersion": "0221", "EXIF DateTimeOriginal": "2012:07:18 10:34:45", "EXIF DateTimeDigitized": "2012:07:18 10:34:45", "EXIF ShutterSpeedValue": "1120723/125000", "EXIF ApertureValue": "126797/20000", "EXIF ExposureBiasValue": "0", "EXIF MaxApertureValue": "4", "EXIF SubjectDistance": "4294967295", "EXIF MeteringMode": "CenterWeightedAverage", "EXIF Flash": "Flash did not fire, compulsory flash mode", "EXIF FocalLength": "24", "EXIF SubSecTime": "14", "EXIF SubSecTimeOriginal": "14", "EXIF SubSecTimeDigitized": "14", "EXIF ColorSpace": "Uncalibrated", "EXIF ExifImageWidth": "500", "EXIF ExifImageLength": "334", "EXIF FocalPlaneXResolution": "5616000/1459", "EXIF FocalPlaneYResolution": "1872000/479", "EXIF FocalPlaneResolutionUnit": "2", "EXIF CustomRendered": "Normal", "EXIF ExposureMode": "Auto Exposure", "EXIF WhiteBalance": "Auto", "EXIF SceneCaptureType": "Standard"}