Lecture 9 Flashcards
What role do rivers play in shaping Earth’s surface?
Rivers are powerful agents of erosion, transport, and deposition. By downcutting channels and moving vast amounts of sediment, they carve landscapes (like canyons) and distribute sediments into basins, influencing Earth’s topography.
How do rivers (streams) begin, and what is a drainage network?
Small rivulets (fed by sheetwash or groundwater) erode and connect to form tributaries. These combine into a drainage network—a system of streams funneling water and sediment to a main trunk stream and eventually larger rivers or bodies of water.
What are some common drainage network patterns, and why do they differ?
Patterns include: Dendritic (branching on uniform bedrock), Radial (streams radiate from a central high), Trellis (streams in alternating resistant/weak rocks), Rectangular (right-angle fractures), and Parallel (steep, uniform slopes). Each pattern reflects underlying geology and topography.
What is a drainage basin (watershed), and what is a drainage divide?
A drainage basin (watershed) is the area where all precipitation drains to a common outlet. A drainage divide is a topographic high (ridge/mountain) separating adjacent basins, sending water to different rivers or oceans.
How is river discharge calculated, and what does it reflect?
Discharge (m³/s) = cross-sectional area (m²) × flow velocity (m/s). The area = width × depth. Discharge reflects the size of the drainage basin and climate; larger or wetter basins tend to have higher discharge.
What is the difference between permanent and ephemeral streams?
Permanent streams flow year-round (e.g., Mississippi), supported by consistent rainfall or groundwater. Ephemeral streams flow only part of the year (often during rainy seasons) and can dry up, common in arid regions.
How do rivers erode their channels?
1) Abrasion: Sediment-laden water grinds the channel. 2) Breaking & lifting: Flow breaks off and lifts clasts. 3) Dissolution: Water chemically dissolves soluble minerals, carrying them away in solution.
What are river competence and capacity?
Competence is the largest particle size a river can move (higher velocity = larger clasts). Capacity is the total quantity of sediment a river can carry, depending on both discharge and competence.
How do rivers transport sediment?
Sediment moves as: Bed load (coarse grains rolling or bouncing), Suspended load (fine silt/clay in the water column), and Dissolved load (ions carried in solution from chemical weathering).
Why does sediment sorting occur as a river’s flow velocity changes?
As velocity decreases, competence drops, causing larger or denser particles to settle first. Over distance and time, sediments become sorted by size, shape, and density.
