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
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
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
Floodplain during dry season, northern Australia Rio Amazonas in flood
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
How do terraces form? period of stability and lateral incision filling and incision glacial outwash climate change increases Q s and/or decreases Q w rise in baselevel due to sealevel increase tectonic uplift at source & increase in coarse sediments vegetation clearing Big Creek, CA southern Israel
northern California
Green River, Utah Narrows Picnic Area, Poudre River, CO
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)
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
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) A f = c A d n n = slope of regression line in log plot Sieve deposits: poorly sorted, lobate form Bajada (alluvial apron), w US
Coastal Peru Annapurna region, Nepal Sinai, Egypt
Pleasant Valley, NV Annapurna region, Nepal
Quito, Ecuador
Pediments erosional bedrock surfaces, often with veneer of sediments generally fan-shaped in plan view 1 km 2 to hundreds of km 2 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
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
Henry Mountains, Utah (pediment first described by GK Gilbert)
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 (Q w, Q s, S, v, w, d) climate tectonics shoreline dynamics
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
Estuary, n California Cook Inlet, coastal Alaska Colorado River delta
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
High-constructive deltas High-destructive deltas Types of deltas (Ritter et al., 1988)
Lena River delta, Siberia
Mississippi River deltas (Schumm, 1977)