Channel Types and Sediments

64. Straight channels — characteristics and cause

Rare in nature. Sinuosity < 1.05. Caused by bedrock control, fault/joint control, or human engineering. Even in "straight" channels, the thalweg (deepest flow path) usually wanders.

65. Thalweg — definition

The line of deepest water in a channel. Wanders back and forth even in straight channels.

66. Meandering channels — characteristics and conditions

Single channel curving in loops across floodplain. Sinuosity > 1.5. Forms on gentle slopes, with fine cohesive sediment (silt/clay banks), steady discharge. Middle to lower reaches of most rivers.

67. Meandering channels — physics of bend formation

Fast surface water thrown to outside of bend (cut bank, erosion); slow water on inside deposits (point bar). Asymmetry makes bends grow larger over time.

68. Braided channels — characteristics and conditions

Single river split into multiple threads separated by mid-channel bars. Sinuosity usually < 1.5. Forms with: high coarse sediment load, variable/flashy discharge, steep gradient, non-cohesive banks (sand/gravel).

69. Braided channels — typical settings

Glacial outwash regions (huge sediment loads from melting ice), mountainous areas with steep gradients and flashy flow. Examples: Alaskan rivers below glaciers, historical Platte River, New Zealand Canterbury Plains.

70. Anastomosing channels — how different from braided?

Also multi-threaded, but separated by PERMANENT VEGETATED islands (unlike braided's ephemeral bars). Forms on very low-gradient reaches with cohesive banks and steady flow, often in wetlands.

71. Sinuosity — formula

Sinuosity = channel length / valley length. Both distances measured between the same two points; only the path differs.

72. Sinuosity — interpretation ranges

1.0 = perfectly straight.

73. Can sinuosity be less than 1?

No — geometrically impossible. Channel length cannot be shorter than the straight-line valley length. Any value <1 is either measurement error or a trick question.

74. Cut bank — definition

Outer bank of a meander bend where fast water erodes. Steep, undercut, actively retreating.

75. Point bar — definition

Inner bank of a meander bend where slow water deposits sand and gravel. Gently sloping, growing outward.

76. Meander wavelength — definition

Distance between corresponding points on successive bends (e.g., cut bank to cut bank across one full "S" curve).

77. Meander amplitude — definition

How far bends swing side-to-side; the width of the meander belt.

78. Meander belt — definition

The strip of floodplain across which the river meanders over time.

79. Oxbow lake — formation

Forms when a meander bend becomes so tight that the narrow neck erodes through (neck cutoff), causing the river to take the shorter path and abandoning the bend as a crescent-shaped lake.

80. Neck cutoff vs. chute cutoff

Neck cutoff: erosion through narrow neck between adjacent bends. Chute cutoff: flood creates a new shorter channel across a point bar. Both shorten the river and create abandoned reaches.

81. Meander scars — definition

Abandoned meander loops visible on the floodplain, sometimes as vegetation differences or slight topography, even after they've filled in with sediment.

82. Why do meandering rivers cut off?

Cutoffs create a shorter, steeper path. Rivers always seek the steeper gradient (lower energy state), so the cutoff captures flow permanently.

83. Braid bars — definition

Sediment deposits between channel threads in braided rivers. Usually unvegetated or sparsely vegetated because they're frequently reworked by floods.

84. How does building a dam affect channel type downstream?

Dams trap sediment. Downstream water becomes "sediment-starved" ("hungry water"), eroding bed and banks (channel incision). Braided channels often shift toward single-thread meandering or incised straight channels.

85. How does deforestation affect channel type?

Increases sediment supply from erosion in the watershed. Can push meandering rivers toward braided (excess sediment overwhelms transport capacity).

86. How does channelization affect a river?

Forces natural meandering rivers into straight artificial paths. Disrupts sediment balance, often causes erosion downstream and deposition problems, damages ecology.

87. Mechanical vs. chemical weathering — key distinction

Mechanical: rock broken into smaller pieces without changing composition. Chemical: rock decomposed by chemical reactions, changing composition. They work together — mechanical increases surface area, accelerating chemical weathering.

88. Frost wedging (freeze-thaw)

Water seeps into cracks, freezes and expands ~9%, prying rock apart. Most important in cold climates and high elevations. Produces angular talus slopes.

89. Thermal expansion (insolation weathering)

Repeated daily heating and cooling causes rocks to expand and contract, eventually cracking. Important in deserts, especially in rocks with mixed mineral composition.

90. Exfoliation (sheeting)

Massive rocks release pressure as overlying material erodes; large sheets spall off parallel to surface. Produces dome landforms like Half Dome or Stone Mountain.

91. Salt weathering (haloclasty)

Salt crystals grow in pores and cracks, exerting pressure. Important in arid regions and coastal areas.

92. Root wedging

Plant roots grow into cracks and expand, wedging rocks apart. A form of biological mechanical weathering.

93. Abrasion

Physical grinding as sediment particles collide during transport (in rivers, glaciers, wind, waves). Rounds particles and reduces grain size.

94. Dissolution — chemical weathering

Minerals dissolve into water. Most important for carbonate rocks (limestone, dolomite) reacting with acidic rainwater. Fundamental to karst topography. Also affects halite and gypsum.

95. Hydrolysis

Water reacts with silicate minerals (especially feldspars) to produce clay minerals + dissolved ions. Most important chemical weathering process for Earth's crust (silicates dominate).

96. Oxidation

Iron-bearing minerals react with oxygen to form iron oxides (rust). Turns rocks red or brown; common in soils.

97. Hydration

Minerals incorporate water into their structure, often expanding and weakening the rock. Anhydrite → gypsum is the classic example.

98. Carbonation

Reaction with CO₂-charged water; a specific type of dissolution central to limestone weathering and karst formation.

99. What controls weathering rates?

Climate (warm+wet accelerates chemical; cold+freeze-thaw accelerates mechanical), rock type (limestone weathers fast, quartzite is resistant), surface area (smaller = faster), vegetation, time.

100. Where does chemical weathering dominate? Where does mechanical dominate?

Chemical dominates in warm, wet climates (tropics). Mechanical dominates in cold or dry climates (mountains, deserts, polar regions).

Topic D: Sediment Sizes and Transport

101. Wentworth scale — from largest to smallest

Boulder (>256 mm), Cobble (64-256 mm), Pebble (4-64 mm), Granule (2-4 mm), Sand (0.0625-2 mm), Silt (0.004-0.0625 mm), Clay (<0.004 mm).

102. What size is sand?

0.0625 mm to 2 mm (1/16 mm to 2 mm).

103. What size is clay?

Less than 0.004 mm (less than 1/256 mm).

104. Boulder threshold size

Greater than 256 mm.

105. Gravel family (informal grouping)

Granules + pebbles + cobbles + boulders (2 mm and up).

106. Mud family (informal grouping)

Silt + clay (less than 0.0625 mm).

107. Bedload — definition and mechanism

Largest particles a stream can move; travels along the bed by traction (rolling/sliding) or saltation (bouncing/hopping). Moves slower than surrounding water. Shapes channel morphology.

108. Traction — definition

Sediment transport mechanism where the largest particles roll or slide along the streambed.

109. Saltation — definition

Sediment transport mechanism where particles bounce along the bed in short hops (from Latin saltare, "to jump"). Typical of sand-sized grains during moderate flow.

110. Suspended load — definition

Small particles (silt, clay, fine sand) lifted into the water column by turbulence, moving essentially with the water. Makes rivers appear muddy/cloudy.

111. Dissolved load — definition

Ions dissolved from chemical weathering (Ca²⁺, HCO₃⁻, Na⁺, Cl⁻, SO₄²⁻) carried in solution. Invisible but significant, especially in limestone regions.

112. Competence — definition

Maximum PARTICLE SIZE a stream can transport. Depends on velocity. High-velocity streams can move boulders; low-velocity streams can only move fine sediment.

113. Capacity — definition

Total AMOUNT (mass) of sediment a stream can transport. Depends on discharge. High-discharge streams carry more; low-discharge streams carry less.

114. Competence vs. capacity — quick distinction

Competence = biggest particle size (velocity-dependent). Capacity = total load (discharge-dependent). A fast, shallow stream can have high competence but low capacity.

115. Sorting — definition

How uniform the grain sizes are in a sediment sample. Well-sorted = uniform sizes; poorly-sorted = mix of sizes.

116. Well-sorted sediment — what does it tell us?

Sediment has been transported far and reworked (beach sand, wind-blown deposits, mature river deposits).

117. Poorly-sorted sediment — what does it tell us?

Short transport distance from source, or deposition by a non-selective agent (glacial till, landslides).

118. Rounding — definition

How rounded individual grains are. Angular = sharp corners (short transport); well-rounded = smooth (long transport with abrasion).

119. Sphericity — definition

How close to spherical a grain is. Different from rounding — an elongated pebble can be well-rounded but not spherical.

120. What does angular + poorly-sorted sediment indicate?

Short transport distance, high-energy or non-selective depositional agent. Examples: landslides, glacial till, sediment close to source.

121. What does well-rounded + well-sorted sediment indicate?

Long transport, lots of reworking. Examples: mature beach, ancient river deposits, wind-blown dune sand.

122. Why is glacial till poorly-sorted and relatively impermeable?

Ice dumps sediment without water sorting it, mixing all grain sizes. Small grains fill spaces between large grains, blocking pore space and reducing permeability.

131. Floodplain — definition and how it builds

Flat area next to a river channel. Builds by overbank deposits (mostly silt and clay from suspended load) during flood events. Vertical accretion over time.

132. Natural levee — formation

Raised bank along a river channel, built by repeated overbank flooding. When water leaves the channel, velocity drops sharply and coarse sediment deposits right at the bank. Over time, builds a natural ridge.

133. Arcuate delta — form and cause

Fan-shaped delta. Dominated by river deposition with moderate wave redistribution. Example: Nile delta.

134. Bird's-foot (elongate) delta — form and cause

Long finger-like distributaries extending into water. Forms where sediment supply is very high and waves are weak. Example: Mississippi delta.

135. Cuspate delta — form and cause

Pointed shape. Wave action smooths the outer edge. Forms where wave energy is high. Example: Tiber delta.

136. Estuarine delta — form and cause

Sediment reworked by tides into elongated ridges parallel to tidal flow. Forms in tide-dominated settings. Example: Ganges-Brahmaputra delta.

137. Alluvial fan — definition and form

Cone-shaped deposit where a steep stream exits a mountain range into a broader valley. As channel spreads out and slope decreases, velocity drops and sediment dumps. Coarse at apex (near mountain), fine at toe. Common in desert Southwest.

138. Lake bed deposits

Fine sediment (silt and clay) accumulates in deep central parts of lakes. Some lakes preserve annual layers (varves) recording climate history.

139. Why do deltas form at river mouths?

Rivers hit standing water; velocity crashes to nearly zero. Everything drops out — coarser sediment first (near the mouth), fine sediment later (further into the basin).