FLUVIAL LANDFORMS Floodplains • fairly flat & continuous surface occupying much of a valley bottom • normally underlain by unconsolidated sediments • subject to periodic flooding (usually once every year or so) • surface & sediments somehow relate to activity of present channel • exerts influence on basin hydrology through lag • serves as sediment storage area 454 lecture 7 Floodplain deposits consist of channel fill: poorly sorted silt, sand & gravel channel lag: coarse materials, fines winnowed splay: breaks in natural levees; coarser than overbank sediments colluvium: near valley sides, slope wash & mass movements lateral accretion: sands & gravels in point bars vertical accretion: silts & clays Meander scrolls form on floodplain of meandering rivers through lateral migration of bends – leave ridges & swales Cutoffs leave oxbow lakes that become clay plugs 454 lecture 7 Floodplains are built primarily by lateral accretion: building point bars up to the floodplain level & then shifting overbank flow & vertical accretion: braided river floodplain is less thick & regular Relative importance of vertical vs lateral accretion varies – vertical is more important where there is frequent flooding & abundant fines In an aggrading river, floodplain sedimentation may exceed the depth to which the river can scour; the sediment is then no longer part of the active floodplain In a degrading channel, the floodplain becomes a terrace when incision prevents the river from flooding the surface every couple of years 454 lecture 7 Floodplain during dry season, northern Australia Rio Amazonas in flood 454 lecture 7 Fluvial terraces abandoned floodplain consist of tread and riser (scarp) can be classified as erosional/depositional strath/alluvial (fill) paired/unpaired tectonic/climatic Difficulties of terrace interpretation (eg. from Cody, Wyoming) Rio Mira, Ecuador 454 lecture 7 How do terraces form? • period of stability and lateral incision • filling and incision glacial outwash climate change increases Qs and/or decreases Qw rise in baselevel due to sealevel increase tectonic uplift at source & increase in coarse sediments vegetation clearing Big Creek, CA southern Israel 454 lecture 7 northern California 454 lecture 7 Green River, Utah Narrows Picnic Area, Poudre River, CO 454 lecture 7 Alluvial Fans and Pediments depositional & erosional features at the base of mountains mountain piedmont (alluvial fans, bajadas, pediments, talus slopes) basin (playa, floodplain) Piedmont consists mainly of fans & pediments (eroded bedrock plains) 454 lecture 7 Alluvial fans fan-shaped in plan view & convex in cross-profile caused by deposition when rivers leave confined channels location of deposition shifts across fan surface with time, both laterally & outward from mountain Adjacent fans coalesce to form bajadas/alluvial aprons Death Valley, CA 454 lecture 7 Fans gradually flatten toward the toe – the steepest areas occur in the upper fan where coarse sediments, low discharge, & sediment transport by mass movements (debris flows) occur: fan profiles tend to be segmented, rather than smooth The area of the fan is related to the source area c = f(climate, lithology, tectonism) n Af = c Ad n = slope of regression line in log plot Sieve deposits: poorly sorted, lobate form Bajada (alluvial apron), w US 454 lecture 7 Coastal Peru Annapurna region, Nepal Sinai, Egypt 454 lecture 7 Annapurna region, Nepal Pleasant Valley, NV 454 lecture 7 Quito, Ecuador 454 lecture 7 Pediments erosional bedrock surfaces, often with veneer of sediments generally fan-shaped in plan view 1 km2 to hundreds of km2 in size convex or concave across the pediment longitudinal profile is slightly concave approximately 2.5° slope dissected by incised channels & dotted with inselbergs (residual bedrock knobs) surface & subsurface weathering, and sheetflow & lateral cutting by channels are all important 454 lecture 7 If pediments erode headward, it could be by a) lateral planation: arid region rivers with coarse loads migrate & erode laterally b) parallel retreat: mountain front achieves equilibrium slope, weathering & erosion maintain slope, and surface retreats parallel to itself c) drainage basin hypothesis: lateral planation dominant along main drainage line, parallel retreat along interfluves Pediment, Mohave Desert, CA 454 lecture 7 Henry Mountains, Utah (pediment first described by GK Gilbert) 454 lecture 7 Deltas Depositional features where river enters a local (lake) or ultimate (ocean) base level At the apex of the delta, the river divides into distributaries – radiating branches that deliver sediment to the extremities Deposition occurs because of a velocity decrease as the river enters a body of standing water Delta form & properties represent adjustment between fluvial system (Qw , Qs , S, v, w, d) climate tectonics shoreline dynamics 454 lecture 7 Basic delta types are high constructive: fluvial action dominates, high sediment input relative to marine dynamics; elongate (more mud) & lobate (more sand) high destructive: ocean or lake energy high, & fluvial sediments are reworked wave-dominated: sediments accumulate as arcuate sand bars tide-dominated: sediments linear The whole delta generally shifts with time 454 lecture 7 Estuary, n California Cook Inlet, coastal Alaska Colorado River delta 454 lecture 7 The interaction of river water with standing water depends on the relative densities of the two: • hyperpycnal flow: inflowing water is denser due to colder temperature or higher sediment concentration – 2d plane jet flow occurs as turbidity current moves along basin floor • homopycnal flow: density of inflowing & standing water are equal – 3d axial jet flow, complete mixing close to river mouth, common in freshwater lake deltas • hypopycnal flow: ocean water is more dense, mixing is slow, river water spreads laterally in plane jet flow 454 lecture 7 High-constructive deltas High-destructive deltas Types of deltas (Ritter et al., 1988) 454 lecture 7 454 lecture 7 454 lecture 7 454 lecture 7 454 lecture 7 Lena River delta, Siberia 454 lecture 7 Mississippi River deltas (Schumm, 1977) 454 lecture 7.