Fluvial Systems – Stream Profiles, Sediment Transport, Channel Patterns & Flow Types

Discharge increases downstream
- Definition: volume of water passing a point per unit time (commonly $m^3/s$ ).
- Main driver = addition of tributaries → more water → larger $Q$ .
- Amazon cited (12–13 major tributaries) → world-record discharge.
Key longitudinal terms
- Headwaters: origin of the channel; typically higher elevation (mountains or uplands, e.g.
  Mississippi headwaters in N. Minnesota).
- Stream mouth: terminus of flow—ocean, sea, or lake (Great Salt Lake example with Jordan River).

Formula: $\text{Gradient}=\frac{\text{vertical drop (rise)}}{\text{horizontal distance (run)}}$ .
- Common units: $m/km$ .
Typical pattern (example values)
1. Near headwaters: steep – $15\,m/km$ .
2. Mid-reach: moderate – $8\,m/km$ .
3. Near mouth: gentle – $4\,m/km$ .
Consequences: higher gradient → faster velocity, more erosive power; low gradient → deposition dominated.

Capacity = maximum sediment volume a stream can move at a given discharge & velocity.
Competence (implied) = largest particle size that can be moved.
Velocity ↑ ⇒ both capacity & competence ↑.
Velocity ↓ ⇒ deposition begins, largest particles drop out first.
Particle-velocity diagram highlights three velocity bands:
1. High velocity
- Clay, silt, fine sand = fully suspended.
- Coarse sand & gravel = transported as bedload (rolling, sliding, saltation).
1. Moderate velocity
- Clay, silt, very fine sand remain suspended.
- Most sand & small gravel = bedload.
- Large gravel begins to deposit.
1. Low velocity
- Only clay/silt stay in suspension.
- Some fine sand = bedload; most sand & all gravel deposited.

Perennial streams
- Flow all year; never run dry.
- Water sources: (1) surface runoff from precipitation, (2) groundwater (aquifer discharge).
- Example: lush, rain-fed stream on Washington’s Olympic Peninsula.
Intermittent streams
- Flow seasonally (wet season) and fall dry in dry season.
- Typical in climates with pronounced wet/dry rhythm (e.g.
  tropical savanna, SW US deserts).
- Runoff is primary supply; limited or unreliable groundwater.
- Example: Cuerco River, AZ/NM border—flow only part of year, cottonwood gallery forests line channel.
Ephemeral streams
- Flow only immediately after rainfall or snowmelt events.
- Dominated by surface runoff; essentially no groundwater input.
- Common in arid basins (Great Basin, Sandia Mts.
  foothills example showing dry sandy channel & line of trees rooted in occasional flow path).

Number of channels
- Single-thread (most common).
- Multi-thread → braided streams (discussed later; many shallow, shifting channels, typical near glaciers or high sediment load).
Sinuosity / Curvature
- Sinuosity Index $=\dfrac{\text{channel length}}{\text{valley (down-valley) length}}$ .
- $S=1$ ⇒ perfectly straight (rare in nature).
- $S>1.5$ typically classified “meandering”.
- Low-sinuosity example: straight, steep streams on E. slope of Mt.
  Hood.
- High-sinuosity example: meandering tributaries of the Willamette or Shenandoah River.

Straight reach cross-section
- Deepest water & highest velocity in centre → minimal friction.
Curved reach
- Highest velocity & deepest water shifts to outside bend → cut bank (active erosion).
- Lowest velocity on inside bend → point bar (active deposition of sand/gravel).
- Alternation of cut banks & point bars on successive bends → lateral migration.
Positive feedbacks
- Erosion at cut bank deepens channel → still faster flow → more erosion.
- Deposition at point bar shallows channel → slower flow → more deposition.

Progressive meander migration widens belt → forms broad floodplain—flat, alluvial surface periodically submerged in floods.
Meander neck: narrow land strip between two adjoining bends.
- During major flood, stream may breach neck, creating a shorter path.
Cut-off results in:
1. New, straighter main channel.
2. Abandoned crescent-shaped water body = oxbow lake.
3. Eventual infilling/vegetation → meander scar.
Real-world illustrations
- Lookout Mountain view of Tennessee River meander near Chattanooga (neck reinforced by Army Corps to prevent cut-off).
- Aerial of Mississippi River long-lots & oxbow lakes (Tunica Lake, LA/AR border) – political boundaries freeze former channel locations.

Rapids
- Localised steepening or channel constriction causes turbulent, aerated flow (“white water”).
- Common in youthful, mountainous rivers.
Waterfalls
- Water drops vertically through air across a nick point.
- Geology: strong caprock overlies weaker strata.
- Weak layer erodes → undercutting → collapse of resistant layer → retreat of falls upstream.
Niagara Falls as case study
- Two falls (Horseshoe & American) on Niagara River.
- Retreat since last glaciation (~12 ka) ≈ $11\,km$ .
- Modern management diverts flow at night for hydroelectric generation, slowing erosion.
Icelandic examples (rapids + falls)
- Skógafoss, Gullfoss, Seljalandsfoss—formed as glacial meltwater drops from volcanic interior plateau to coastal plain.

Engineering intervention
- Cut-bank revetments & levees (e.g.
  Chattanooga, lower Mississippi) to protect property & navigation, but alter natural meander evolution and habitat.
Land survey legacies
- French long-lot system oriented perpendicular to rivers—maximises frontage and persists around former meanders & oxbows (South Louisiana, Québec).
Tourism vs. geomorphic integrity
- Niagara Falls flow diversion balances power generation, tourist aesthetics, and erosion control—an ongoing management compromise.

Gradient: $G=\dfrac{\Delta h}{\Delta l}$ ; values along profile: $15, 8, 4\,m/km$ .
Discharge (implied): $Q=A\,v$ .
Sinuosity: $S=\dfrac{L<em>{\text{channel}}}{L</em>{\text{valley}}}$ .
Velocity–size relationships shown qualitatively on Hjulström-type diagram (thresholds for erosion, transport, deposition).

How does increasing tributary density influence downstream flood hazard?
Contrast the sediment signatures you would expect to core from point bars vs.
oxbow-lake fills.
Evaluate human alterations (levees, revetments, flow diversions) in terms of their impact on cut-off frequency and floodplain ecology.