Geomorphology: River Systems, Drainage Basins, and Fluvial Landforms

Comprehensive practice flashcards covering river geomorphology, hydrological system components, drainage basin dynamics, stream order calculations, and fluvial landforms based on lecture notes.

How do impermeable surfaces like tarmac and concrete alter the water cycle in urban areas?

Impermeable surfaces prevent water infiltration into the soil, reducing groundwater recharge and increasing surface runoff, which is usually directed through artificial drains.

How does deforestation impact natural water storage and river flooding risk?

Clearing vegetation removes trees and plants that intercept and store water, reducing absorption and evapotranspiration, which leads to increased runoff and higher flood risk.

What is an interfluve in a drainage basin system?

An interfluve is the raised area of land located between two adjacent tributaries.

What are the definitions of a river's source, mouth, confluence, and watershed?

The source is where a river starts; the mouth is where it enters the sea or a lake; a confluence is the point where two rivers meet; and a watershed is the high ground boundary separating two adjacent drainage basins.

Which environmental conditions result in decreased runoff and increased infiltration?

A gentle slope, dense vegetation cover, permeable rock (like sandstone), porous or sandy soil, dry soil moisture, high evaporation rates, and soft, soaking rainfall.

How are permanent, periodic, episodic, and exotic rivers classified based on flow and the water table?

Permanent (perennial) rivers flow all year because the water table is always above the riverbed. Periodic (non-perennial) rivers flow only during the rainy season when the water table rises above the riverbed. Episodic rivers flow briefly only after heavy rain episodes while the water table remains below the riverbed. Exotic rivers originate in high-rainfall regions and flow through dry, arid areas (such as the Orange River).

What are isohyets?

Isohyets are lines on a map connecting points that receive equal amounts of rainfall.

What rock and landscape structures form dendritic, radial, trellis, and rectangular drainage patterns?

Dendritic patterns form on flat, uniformly resistant bedrock. Radial patterns form when streams flow outward from a high central point like a volcano or dome. Trellis patterns form in parallel fold mountains where tributaries join main streams at $90^\circ$ angles. Rectangular patterns develop along rectangular joint systems on rocks like granite.

What is the formula for calculating drainage density?

$$\text{Drainage density} = \frac{\text{Total length of streams in the basin } (\text{km})}{\text{Total area of the drainage basin } (\text{km}^2)}$$

How do rock type and vegetation cover affect drainage density?

Impermeable rock (like granite) and sparse or cleared vegetation lead to less infiltration, increased runoff, and higher drainage density. Permeable rock (like sandstone) and dense vegetation promote water absorption, leading to less runoff and lower drainage density.

How is stream order calculated in a river basin network?

All unbranched source streams are assigned a $1^{\text{st}}$ order. When two streams of the same order meet (e.g., two $1^{\text{st}}$ order streams), they join to form a higher order stream ($2^{\text{nd}}$ order). When a lower order stream joins a higher order stream, the river retains the higher order number.

What is the key functional difference between laminar flow and turbulent flow in a river?

Laminar flow occurs over flat, smooth riverbeds where water flows smoothly in uniform sheets with minimal friction. Turbulent flow occurs over rough, uneven beds where obstacles create friction, slowing down water and causing bubbly, white-water motion.

What distinguishes a superimposed river from an antecedent river?

A superimposed river is younger than the landscape underneath and erodes downward through surface rock into an underlying, older geological structure. An antecedent river is older than the surrounding landscape and maintained its original flow course by eroding through rock layers as tectonic compression or uplift created new ridges.

What is a longitudinal profile, and how do temporary base levels differ from a permanent base level?

A longitudinal profile shows the side view of a river's gradient from source to mouth, typically forming a smooth concave curve. A permanent base level is the lowest elevation to which a river can erode (sea level). A temporary base level is a local obstacle to erosion (such as a dam or waterfall) below which the river cannot erode until the feature is worn away.

How does the transverse profile of a river valley change from its upper to its lower course?

In the upper course, the valley has a steep-sided, narrow V-shape created by dominant vertical erosion. In the middle course, lateral erosion widens the valley into an open U/V-shape with a narrow floodplain. In the lower course, the transverse profile becomes a very wide, flat-bottomed valley dominated by deposition and bounded by levees.

What are the four primary mechanisms of fluvial erosion?

1. Abrasion: sediment carried by the river scours the channel bed and banks like sandpaper. 2. Hydraulic Action: the force of moving water forces air into rock cracks, expanding them until they break. 3. Attrition: rocks and boulders carried by the water collide with each other, breaking into smaller, smoother pebbles. 4. Solution (Corrosion): mild acids in the water dissolve soluble rock types along the channel.

How are potholes formed in a riverbed?

Potholes form through pothole scouring, where loose pebbles get caught in small depressions on the rocky riverbed and spin around in turbulent currents, grinding out deep circular hollows via abrasion (such as Bourke's Luck Potholes).

What five transport mechanisms move a river's load downstream?

1. Traction: rolling or dragging large boulders along the riverbed. 2. Saltation: bouncing smaller pebbles along the channel bed. 3. Suspension: carrying fine silt and clay particles suspended in muddy water. 4. Solution: carrying dissolved minerals in water. 5. Float: carrying light debris floating on the surface.

What hydraulic processes occur on the outer bend versus the inner bend of a meander?

The outer bend experiences the fastest flow velocity, driving lateral erosion and undercutting that forms a steep river cliff and deep water. The inner bend experiences the slowest flow velocity, leading to sediment deposition that forms a shallow slip-off slope.

What are the five main steps in the formation and retreat of a waterfall?

1. The river undercuts soft rock underlying an upper hard rock layer. 2. The unsupported hard rock overhang eventually collapses. 3. Fallen debris and falling water scour out a deep plunge pool at the base. 4. The waterfall process repeats, causing the waterfall to retreat upstream. 5. A steep-sided gorge is left downstream as the waterfall recedes.

How do rapids form along a river channel?

Rapids form in the upper course where a river flows over alternating thin bands of hard and soft rock. The softer rock erodes faster, leaving a sequence of resistant hard rock steps that trigger fast, turbulent white-water flow.

What causes the formation of a braided stream pattern?

A braided stream forms when a river carrying a heavy sediment load loses velocity and deposits alluvium within its channel. These deposits form sandbars and river islands, forcing the river to split into multiple small, interconnecting channels.

What is an alluvial fan and under what conditions does it form?

An alluvial fan is a fan-shaped accumulation of sediment deposited where a steep mountain stream flows out onto a flat plain, rapidly losing velocity and spreading its load.

How does an oxbow lake form, and what is a meander scar?

An oxbow lake forms when erosion narrows the neck of an extreme meander loop until the river cuts directly through the neck during a flood. Deposition eventually seals off the abandoned loop, leaving a crescent-shaped lake. When an oxbow lake completely dries up, it leaves behind a dried depression called a meander scar.

How do natural levees form along river banks during repeated flooding?

When a river overflows its banks, water velocity drops immediately outside the main channel. Coarse, heavy sediments are deposited right along the channel edges, while fine silt spreads across the floodplain. Over multiple flood events, these coarse deposits build up into raised natural embankments called levees.

What is a delta and where does it form?

A delta is a large accumulation of sediment deposited at the mouth of a river where it enters a standing body of water (such as a sea or lake), often causing the main channel to split into smaller distributaries.