A valley is an elongated depression or low area between higher areas such as hills, mountains, plateaus or ridges. Valleys can contain rivers, streams, lakes, wetlands, roads, settlements and agricultural land, although water is not required for a feature to be classified as a valley. Their shapes and origins vary greatly because valleys can be produced by running water, glaciers, tectonic movement, mass wasting, volcanic activity, erosion and combinations of these processes. Valley is therefore a broad geographical term describing a landform rather than one specific method of formation.
River valleys develop principally through erosion by flowing water and associated processes. A river cuts downward into its bed while also eroding laterally into its banks and surrounding sediments. In upper courses, steep gradients can favour vertical incision and produce narrow V shaped valleys. Farther downstream, reduced gradients and greater lateral migration can create wider valleys with floodplains, meanders and terraces. Weathering and mass movement continuously modify valley sides. The resulting form reflects the balance between river erosion, sediment deposition, rock resistance, climate, uplift and changes in base level.
Glacial valleys have a different characteristic shape. A moving glacier occupies and enlarges an existing valley through abrasion and plucking, commonly producing a broad U shaped cross section with relatively steep sides and a wide floor. Hanging valleys can occur where tributary glaciers occupied smaller valleys that join a deeper main glacial trough. Glacial valleys may contain lakes, moraines, waterfalls and other glacial deposits. The contrast between V shaped fluvial valleys and U shaped glacial valleys is useful, although natural landscapes can contain transitional or modified forms.
Tectonic valleys can develop when crustal movement creates faults, fractures or zones of subsidence. A graben, for example, is a down dropped block bounded by faults and may become occupied by a river or lake. Volcanic activity can also produce valleys through lava erosion, collapse or the removal of weaker materials. Coastal valleys may be drowned by rising sea level and become estuaries or fjords depending on their origin and morphology. Consequently, valley shape alone should not always be treated as definitive evidence of a single formative process.
Valley floors commonly accumulate sediment because water and ice transport material from higher terrain and deposit it where transport capacity decreases. River valleys can contain alluvium, including gravel, sand, silt and clay, while glacial valleys can contain till and outwash. Floodplains are especially important valley floor features in mature river systems. Terraces may represent former valley floors abandoned as a river incises downward. The arrangement of these deposits can reveal changes in discharge, sediment supply, vegetation, climate, tectonic uplift and downstream base level through geological time.
Valleys influence human geography because they often provide relatively accessible routes through mountainous terrain and sources of water, fertile soils and settlement space. Roads, railways and rivers frequently follow valley corridors because topographic gradients are generally lower than on adjacent slopes. However, valley settlements can face hazards such as flooding, landslides, avalanches, debris flows and, in some regions, volcanic or seismic activity. The suitability of a valley for development therefore depends on its geology, hydrology, slope stability, climate and the frequency and magnitude of natural hazards.
A valley should be distinguished from a gorge, canyon, ravine and basin. A gorge or canyon is generally a deeply incised, steep sided form, whereas valley is the wider generic category. A basin is normally a broader depression that may drain internally or externally and need not have the elongated form characteristic of many valleys. A ravine usually denotes a smaller narrow valley or erosion channel. These terms overlap in everyday usage, and regional terminology can differ. Physical interpretation should therefore consider measurable characteristics such as width, depth, slope, drainage and geological structure.
Valleys are important records of landscape evolution because their geometry reflects the interaction of erosion and deposition over time. A valley may begin as a narrow incision and become progressively wider through lateral erosion and slope retreat. Later uplift or renewed lowering of base level can cause renewed incision, leaving terraces or nested valley forms. Climate change can alter vegetation, runoff, glacier extent and sediment supply, modifying valley processes. Modern geomorphology therefore treats valleys as dynamic landforms whose present appearance represents the cumulative effects of geological structure, climate, hydrology, erosion and deposition.
