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Explores how running water shapes the landscape through erosion, transport, and deposition.
Why does a river, seemingly following the path of least resistance, choose to twist into complex loops rather than flowing in a straight line to the sea?
The Hjulstrom Curve is a fundamental graph used to show the relationship between the velocity of a river and the size of the particles it can erode, transport, or deposit. The x-axis represents particle size () in millimeters, while the y-axis represents velocity () in . A unique feature of the curve is the 'clay anomaly': while larger particles like gravel require high velocities to erode due to their weight, very fine clay and silt also require high velocities because they are cohesive (they stick together). Once eroded, however, these fine particles can be transported at extremely low velocities, often remaining in suspension until the water is almost still.
Suppose a river's velocity is measured at . 1. Locate on the y-axis of a Hjulstrom Curve. 2. Observe that for a particle of (sand), this velocity is well above the critical erosion velocity. 3. Result: The sand will be eroded from the bed and transported downstream.
Quick Check
Why does it take more energy to erode clay than sand, despite clay particles being smaller?
Answer
Clay particles are cohesive (sticky), requiring higher energy to break their molecular bonds and lift them into the flow.
Rivers rarely flow straight because of helicoidal flow—a corkscrew-like motion of water. The fastest current, the thalweg, swings toward the outer bank of a bend, causing lateral erosion through hydraulic action and abrasion. This creates a steep river cliff. On the inner bank, velocity is lower, leading to the deposition of sediment and the formation of a point bar or slip-off slope. Over time, the neck of the meander narrows. During a flood, the river may cut through the neck to take a shorter, steeper path. Deposition eventually seals off the old loop, creating an oxbow lake.
Consider a meander where the neck width is decreasing at a rate of . 1. If the current neck is wide, in 20 years, the river is likely to breach the neck during a high-discharge event. 2. The breach creates a 'new' straight channel. 3. Reduced energy in the old loop leads to siltation at the ends, isolating the water body.
Quick Check
Where is the thalweg located in a meander, and what landform does it create?
Answer
The thalweg is on the outside of the bend, and it creates a river cliff through erosion.
The geometric arrangement of streams in a region, known as the drainage pattern, is primarily determined by the underlying rock type and structure. Dendritic patterns look like tree branches and form on uniform (homogeneous) rock. Trellis patterns, where short tributaries feed into main rivers at right angles, occur in areas of alternating hard and soft rock (folded strata). Radial patterns form when streams flow outward from a central high point, like a volcanic dome, while Rectangular patterns follow joints or faults in the bedrock that meet at sharp angles.
Imagine an area with parallel ridges of resistant quartzite and valleys of soft shale. 1. Streams will naturally erode the softer shale, forming long, parallel main channels. 2. Short tributaries will flow down the quartzite ridges to join the main stream at . 3. This identifies the pattern as Trellis, indicating a landscape of folded sedimentary rock.
According to the Hjulstrom Curve, what happens to a pebble if the velocity drops from to ?
Which process is responsible for the formation of a point bar?
A radial drainage pattern is most likely to be found on a flat plain with uniform soil composition.
Review Tomorrow
In 24 hours, try to sketch the Hjulstrom curve from memory, specifically noting why the erosion line rises for the smallest particles.
Practice Activity
Use Google Earth to find the Mississippi River or the Amazon River. Identify three oxbow lakes and one instance of a dendritic drainage pattern.