Discuss the role of Slope, Altitude and Relief (SAR) in landscape development.

Discuss the role of Slope, Altitude and Relief (SAR) in landscape development. (2022)

  • Slope is the inclination of the ground surface; altitude is height above mean sea level; relief is the vertical difference between high and low points within a unit area — relative (local) relief or “amplitude of available relief”, as against absolute relief, height above mean sea level.
  • Together they fix the potential energy of a landscape: relief and altitude above base level set how much can be eroded, altitude sets which climatic processes operate, and slope sets how fast and by what mechanism material moves.
  • They are both controls on landscape development and products of it, which is why morphometry measures all three.

Slope — Where Denudation Is Expressed

  • Slope sets the gravitational component acting on regolith and water, and therefore runoff velocity, infiltration and the type of mass movement: rockfall and landslides on steep slopes, creep and solifluction on moderate ones, sheetwash and deposition on gentle ones.
  • Alan Wood (1942) divided a full hillslope into four elements — waxing slope (convex crest), free face, constant (debris) slope and waning slope (concave base) — a scheme Lester Charles King adopted for his scarp-retreat model.
The four element composite slope profile
  • How slopes change through time defines the classic models of landscape evolution:
    • William Morris Davis — slope decline, angles flattening with time;
    • Walther Penck — slope replacement, gentler units extending upslope at the expense of steeper ones;
    • Lester Charles King — parallel retreat of the free face, leaving a widening pediment.
  • Slopes cannot steepen indefinitely: beyond a threshold angle set by rock and regolith strength, landslides take over and slopes stop steepening even as uplift continues — Arthur Newell Strahler’s “repose slopes” are the debris-controlled form of this limit.
  • Aspect adds a second control: in mid-latitude and Himalayan valleys the sun-facing and shaded sides differ in freeze–thaw, vegetation and moisture, producing asymmetric valleys.

Altitude — The Vertical Control on Process

  • Temperature falls about 6.5 °C per 1,000 m of ascent, so altitude zones the processes: running water and chemical weathering on lower slopes, periglacial frost action and solifluction above the treeline, glacial erosion above the snowline — fluvial, periglacial and glacial landforms on one Himalayan transect.
  • Altitude above base level is the store of energy the rivers spend; denudation lowers it, and isostatic rebound partly restores it, so mountains lose height more slowly than they lose rock.
  • Altitude also governs orographic rainfall: the windward Western Ghats receive heavy rain and are deeply dissected, while the rain-shadow Deccan interior is more gently eroded.
  • The “glacial buzzsaw” hypothesis holds that glaciers near the snowline plane off peaks and cap mountain height; Ian S. Evans (2017) showed it is not universal, since low-gradient surfaces survive above cirques in several ranges.

Relief — The Energy Budget and the Clock

  • Relative relief measures the depth of dissection: high relief means steep channel gradients, high stream power, dense drainage and frequent mass wasting — the Himalaya; low relief means sluggish rivers and lateral erosion — the Indo-Gangetic plain and much of the Peninsular plateau.
  • In William Morris Davis’s cycle relief also marks stage: during youth the divides barely lower while valleys deepen, so relative relief rises; it peaks around early maturity, then falls through old age towards a peneplain.
Line diagram of summit and valley-floor altitude through youth, maturity and old age, showing relative relief rising then falling to a peneplain.
  • Walther Penck linked relief instead to the rate of uplift relative to erosion: relief grows while uplift outpaces incision (waxing development) and declines when uplift slows (waning development).
  • Recent work quantifies the link: Byron A. Adams, Kelin X. Whipple and colleagues (2020) showed that erosion rate rises non-linearly with fluvial relief, the proportionality set by mean annual rainfall, and confirmed the relation in the Himalaya.

SAR as One System

  • Uplift raises altitude, increases relief and steepens slopes; steeper slopes and higher relief speed denudation, which lowers relief — a negative feedback behind every cyclic and equilibrium model.
  • Where uplift and erosion balance, slopes and relief can hold a steady form — the dynamic equilibrium of John Tilton Hack (1960).
  • Arthur Newell Strahler’s hypsometric (area–altitude) analysis (1952) turns the triad into a single index: a high hypsometric integral marks a little-eroded, “youthful” basin, a low one a deeply denuded basin.
  • SAR maps drawn from digital elevation models now underpin terrain classification and landslide hazard zonation in the Himalaya and Western Ghats.
  • Judgement: relief supplies the energy, altitude chooses the processes and slope expresses them, and all three adjust together — but they are dependent as well as independent variables, set by uplift, rock strength and climate, so SAR explain landscape development best when read alongside tectonics and rainfall rather than as autonomous causes.