6.3 River systems and watershed dynamics

Watershed Characteristics

Watershed and Drainage Basin

A watershed is an area of land that drains water, sediment, and dissolved materials to a common outlet along a stream channel. You'll also see it called a drainage basin, catchment, or river basin. The Amazon River basin and Mississippi River basin are large-scale examples.

Watersheds are bounded by topographic divides, which are ridgelines or elevated terrain that separate surface water flow between two different drainage systems. Rain falling on one side of a divide flows toward one outlet; rain on the other side flows toward a different one.

Watersheds exist at many scales. A small stream has its own watershed, and that watershed sits inside the larger watershed of the river it feeds into. This nesting pattern means smaller watersheds are always contained within larger ones.

Stream Order and River Profile

Stream order (specifically the Strahler system) classifies streams by their relative size and position within a river network. The system works like this:

1. First-order streams are the smallest unbranched tributaries, typically headwater streams with no other streams flowing into them.
2. When two first-order streams merge, they form a second-order stream. Two second-order streams merge to form a third-order stream, and so on.
3. Stream order only increases when two streams of the same order combine. A first-order stream joining a second-order stream does not change the order of the larger stream.

Higher-order streams generally have greater discharge, wider channels, and lower gradients. The Mississippi River, for reference, is a tenth-order stream.

The river profile (or longitudinal profile) is a cross-section of a river from source to mouth, showing how the gradient changes along its length.

• Profiles are typically concave: steep near the headwaters and progressively gentler toward the mouth.
• Underlying geology shapes the profile. Resistant rock layers create steeper sections (sometimes producing waterfalls or rapids), while softer rocks produce gentler gradients.
• A river whose profile has reached a smooth, uninterrupted concave shape is said to be at a graded condition, meaning erosion and deposition are roughly in balance along its length.

Base level is the lowest elevation to which a river can erode its channel. For rivers flowing to the ocean, ultimate base level is sea level. Temporary (or local) base levels also exist, such as a resistant rock layer or a lake that limits how far a river can cut down.

When base level changes, river behavior responds. A drop in sea level causes a river to erode more aggressively and cut deeper into its channel (this is called rejuvenation). A rise in base level has the opposite effect, promoting sediment deposition and channel aggradation.

River Hydrology

Discharge and Erosion

Discharge is the volume of water flowing through a river channel per unit time, measured in cubic meters per second ($m^3/s$) or cubic feet per second ($\text{cfs}$). It's calculated as:

$Q = A \times v$

where $Q$ is discharge, $A$ is the cross-sectional area of the channel, and $v$ is the average flow velocity.

Discharge varies over time with changes in precipitation, snowmelt, and groundwater inputs, and it generally increases downstream as tributaries join the main channel and the drainage area grows. A hydrograph plots discharge against time for a given point on a river, and it's the main tool for tracking how a river responds to storm events or seasonal changes.

Rivers erode their channels and transport sediment through three main processes:

• Hydraulic action: The sheer force of moving water pushes against the channel bed and banks, dislodging and transporting material.
• Abrasion: Sediment particles carried by the river collide with each other and scrape against the channel bed and banks, physically wearing them down. This is the dominant erosion process in most rivers.
• Dissolution: Slightly acidic river water chemically dissolves soluble rock, particularly limestone and similar carbonate minerals.

Sediment Transport in Rivers

Rivers carry sediment in three forms:

• Dissolved load: Ions and molecules dissolved in the water, produced by chemical weathering of rocks throughout the watershed. You can't see this material; it's in solution.
• Suspended load: Fine particles (silt and clay) held up within the water column by turbulence. This is what gives muddy rivers their color, and it typically makes up the largest proportion of a river's total sediment load.
• Bedload: Larger particles (sand and gravel) that move along the river bottom by rolling, sliding, or saltating (bouncing in short hops). Bedload is harder to measure but usually accounts for 5–10% of total sediment transport.

A river's ability to transport sediment depends on two related but distinct properties. Competence refers to the largest particle size a river can move, which depends primarily on velocity. Capacity is the total amount of sediment a river can carry, which depends on both discharge and velocity. This is why seasonal flood events carry far more sediment than low-flow periods: both competence and capacity spike during high flows.

Channel and Floodplain Dynamics

Channel Morphology and Floodplains

Channel morphology describes the shape and form of a river channel, including its width, depth, sinuosity (how curvy it is), and gradient. Morphology reflects the balance between water flow, sediment supply, and the resistance of the bed and bank materials. Depending on these factors, rivers develop different channel patterns:

• Straight channels are relatively rare and usually short. Even in straight channels, the deepest part of the flow (the thalweg) tends to wander from side to side.
• Meandering channels have a winding, sinuous path. These are common in low-gradient settings with cohesive bank materials and moderate sediment loads.
• Braided channels split into multiple interwoven threads separated by bars of sediment. These form where sediment supply is high relative to discharge, like the Brahmaputra River or glacial outwash streams.

Floodplains are the relatively flat areas flanking a river channel that get periodically covered by floodwaters. They form through repeated sediment deposition during floods, building up layers of fertile alluvial soil. Floodplains serve several functions:

• They slow and store floodwaters (flood attenuation), reducing peak discharge downstream.
• They recharge groundwater as floodwaters soak into the ground.
• They provide habitat for riparian ecosystems, the plant and animal communities along river corridors.

The Nile and Mississippi River floodplains are well-known examples. Within floodplains, you'll often find natural levees, low ridges of coarser sediment deposited right along the channel banks where floodwater first spills over and loses velocity.

Meandering Rivers

Meandering rivers develop sinuous, curving channels, typically in low-gradient environments with plenty of fine sediment and cohesive (erosion-resistant) bank materials. The key to understanding meanders is the asymmetry of flow around each bend:

• Water moves faster on the outside of a bend, eroding the bank and creating a steep cut bank.
• Water moves slower on the inside of a bend, depositing sediment and building a gently sloping point bar.

A secondary circulation pattern called helical flow reinforces this process. Water spirals within the channel, moving along the surface toward the cut bank and along the bottom toward the point bar, continuously transporting eroded material from the outside to the inside of the bend.

This pattern causes meander migration: the river gradually shifts laterally as it erodes outward and deposits inward. Over time, a meander loop can become so exaggerated that the river cuts through the narrow neck of land, abandoning the loop. The abandoned bend becomes an oxbow lake, a crescent-shaped body of water separated from the main channel. The Mississippi River has many well-known oxbow lakes formed this way.

Meandering rivers also create a variety of landforms and habitats, including scroll bars (ridges marking former point bar positions), chute channels, and backwater areas. These features support diverse floodplain ecosystems and riparian biodiversity, which is why intact meandering river systems are so ecologically valuable.