Chapter 13 Lecture 5 Notes – Alluvial Fans, Braided Streams, Deltas & Low-Gradient Stream Landforms

Lecture 5 for Chapter 13 (final segment).
Topics sequenced as:
1. Alluvial Fans
2. Braided Streams
3. Deltas
4. Suite of landforms tied to low-gradient streams (meanders, point bars, cut-banks, flood-plains, oxbow lakes, meander scars, stream terraces, Yazoo streams).
Purpose: consolidate depositional & erosional features first introduced earlier in Ch. 13 and cross-link to later glacier & coastal chapters (glacial melt-water; base-level & sea-level change).

Definition & Process
- Depositional landform produced when a confined high-velocity stream exits a narrow canyon and spreads onto a flat surface.
- Stream type: chiefly ephemeral (flow only after precipitation/snowmelt) or occasionally intermittent; discharge is flashy and short-lived.
- Hydraulic shift: confinement lost → channel widens → $v$ (velocity) & competence drop → sediment dumped in a radial, fan-shaped prism.
- Sediment package is called alluvium (mix of clay, silt, sand, gravel, cobbles, even boulders).
Environmental Setting
- Arid/semi-arid mountain fronts dominate (e.g., Mojave Desert near Death Valley, CA).
- High-latitude deserts (cold, low precipitation) also host fans; ex: proglacial canyons in Greenland.
Morphology & Dimensions
- Apex at canyon mouth; radial distributary channels; down-fan decrease in grain size.
- Typical slopes: $1^{\circ}$ – $6^{\circ}$ ; radius from a few 100 m to >10 km depending on catchment size.
Significance / Applications
- Hazard: debris-flow & flash-flood risk on fans.
- Aquifers: coarse strata provide high-yield groundwater.
- Paleoclimate archives: fan stratigraphy records pluvial cycles.

Visual Identity
- Multiple intertwining channels separated by transient bars/islands of sand & gravel; no single, stable thalweg.
Key Controls
1. High sediment load relative to discharge (bed-load dominated).
2. Rapid variations in discharge (snowmelt, storm bursts, glacial melt).
3. Broad, low-gradient valley floor that permits lateral channel migration without deep incision.
Typical Setting
- Proximal to melting alpine or continental glaciers.
- Example sequence (Iceland TikTok clip):
  • Glacier snout → steep melt-water channel (competence high)
  • Reaches coastal/sandar plain → slope flattens → coarse outwash deposited → braided network develops.
Sedimentary & Fluvial Dynamics
- Bedload lobes are deposited mid-channel; subsequent floods bifurcate around them.
- Channels avulse frequently; individual bars may vegetate if flood-free for several seasons.
Connections to Later Topics
- Chapter on Glaciation: outwash plains, eskers & kames originate from similar hydraulics.
- Flood hazards: sudden jökulhlaups create extreme braided re-works.

Definition & Base-Level Concept
- Delta forms where a river reaches its base level (lowest elevation to which it can erode) and enters standing water (ocean, sea, lake).
- Drop in velocity ( $v \downarrow$ ) triggers deposition of suspended & bedload.
Morphological Types (after Galloway/Wright):
- River-dominated (e.g., Mississippi “bird-foot”).
- Wave-dominated (e.g., Nile classic triangle – term “delta” from Greek letter $\Delta$ ).
- Tide-dominated (e.g., Ganges–Brahmaputra).

Fan vs. Delta – Key Contrasts

Parameter	Alluvial Fan	Delta
Stream size	Typically small, ephemeral	Large, perennial
Location	Mountain front interior	At river mouth (base level)
Containing medium	Air (sub-aerial)	Standing water (subaqueous pro-delta)

Nutrient & Population Angle
- Fresh, frequently renewed alluvium → fertile soils → historically dense populations.
- Examples:
  • Nile Delta: cradle of Egyptian agriculture.
  • Ganges–Brahmaputra: among highest global population densities.
  • Mekong Delta (Vietnam): rice bowl; Google-Earth imagery shows near-continuous field mosaics.
  • Nisqually River Delta (WA): much smaller, illustrates glacial stream → delta continuum.
Hazards & Management
- Subsidence, compaction & sea-level rise.
- Levee confinement starves delta lobes of new sediment (Mississippi case study).
- Salt-water intrusion affects agriculture & aquifers.

Context
- Occur on low-relief plains where gradient $S = \Delta h / \Delta l$ is small.
- Classic reach: lower Mississippi River south of its Missouri & Ohio confluences.

Sinuous loops generated by helicoidal flow; migrate laterally.
Erosion on cut-bank (outer bend, high $v$ ); deposition on point bar (inner bend, low $v$ ).

Flat, alluvium-filled surfaces adjacent to channel; inundated during high-flow.
Example: Brazos River, TX:
• South of Hwy-21: floodplain west of river.
• North of Hwy-21: floodplain east – controlled by local bedrock bluffs.

When a meander neck narrows and cuts off (avulsion during flood), the abandoned loop becomes an oxbow lake; eventually silts in → meander scar.

Crescent-shaped vegetation or moisture anomalies (satellite imagery south of Memphis, TN); relic of former oxbows.

Definition: stranded, older floodplain surfaces perched above current one.
Two origins:
1. Incision of channel (base-level fall or increased discharge).
2. Regional uplift raising former floodplain.
Geometry: can appear on one or both banks.
Memphis Bluffs example: terrace on east bank (TN) towering above Mississippi; west bank (AR) remains planar floodplain.

Concept: tributary that runs parallel to main river for tens of miles, separated by natural levee or subtle ridge, before finally joining.
Name derived from Yazoo River, NW Mississippi.
Example pair:
• Yazoo River & Big Sunflower River flow ~ $50$ – $100$ mi south beside the Mississippi before breaching levee near Vicksburg.
Geomorphic cause: natural levees bordering Mississippi stand slightly higher than adjoining backswamps, blocking direct tributary entry.

Agricultural reliance on flood-built soils raises ethical questions of settlement in hazard-prone deltas & floodplains (subsidence, levee failure).
Water-resource management:
• Alluvial-fan groundwater vs. contamination from arid-land development.
• Deltaic subsidence and climate-change adaptation (e.g., Mekong & Nile plans).
Conservation: braided-stream habitats (e.g., Icelandic sandar) critical for certain bird species; human diversion & hydroelectric dams alter sediment budgets.

Stream gradient: $S = \dfrac{\Delta h}{\Delta l}$ .
Competence proportional to $v^{2}$ ; drops sharply when channel widens at canyon mouth or river mouth (fan & delta genesis).
Typical Yazoo parallel reach length: $50$ – $100\;\text{mi}$ (80–160 km).

Ch. 12 (Weathering & Mass Wasting): debris flows feed alluvial fans.
Ch. 14 (Glaciers): melt-water outwash → braided streams; links Langjökull, Iceland.
Ch. 15 (Coasts): deltaic interaction with waves/tides shapes shoreline evolution.

Alluvial Fan = canyon mouth + ephemeral floods + arid mountains.
Braided Stream = high bedload + variable discharge + flat valley (often glacial).
Delta = river meets standing water (base level); fertile, populous, hazard-laden.
Low-Gradient Plains host: meanders → oxbows → scars; floodplains; terraces; Yazoo tributaries.

Study this cascade from high-energy mountain processes to low-energy plains to connect geomorphic systems across spatial scales.