Lecture 10 Flashcards
What is the Hjulström diagram, and how is it used?
The Hjulström diagram plots water velocity against sediment grain size to indicate whether a river of a given velocity will erode, transport, or deposit grains. Coarser grains need higher velocity to erode or remain in suspension, while finer grains can stay in suspension at lower velocities but also need very low flow speeds to settle.
How do we define the gradient of a stream, and why is it important?
Gradient is the change in elevation over a given horizontal distance (slope). A higher gradient means faster flow and stronger erosion near headwaters. A lower gradient near the mouth leads to slower flow and increased deposition.
What landforms result from river downcutting in steep terrains?
Rivers carve canyons (steep-walled troughs) or valleys (gentler slopes). Canyon or valley shape depends on bedrock resistance, climate, and flow intensity.
Why are waterfalls considered temporary stream features?
A waterfall forms over a resistant ledge or drop. Ongoing headward erosion (abrasion and undercutting) gradually migrates the waterfall upstream, eventually collapsing the ledge and eliminating the sharp drop.
How do braided streams differ from meandering streams?
Braided streams have multiple, interweaving channels laden with coarse sediment, often from fluctuating flow in high-sediment regions. Meandering streams have a single sinuous channel in lower-gradient settings, focusing on suspended load with point bars on inside bends and cut banks on outside bends.
Why do meandering streams curve, and what features do they form?
Small channel irregularities cause faster flow on one side (erosion at cut bank) and slower flow on the other (deposition at point bar). This feedback grows meanders over time, creating floodplains and oxbow lakes from abandoned loops.
What is a delta, and why do they form at the mouths of rivers?
A delta is a sediment wedge deposited where a river enters a standing body of water. As flow velocity abruptly slows, coarser sediment settles first, then finer material. Over time, branching distributaries build the delta outward.
What leads to flooding, and what are some common causes?
Flooding happens when a river’s discharge exceeds channel capacity. Causes include intense/prolonged rainfall, snowmelt, dam or levee failures, and saturated or frozen ground limiting infiltration.
Distinguish between seasonal (slow-onset) floods and flash floods.
Seasonal floods develop over days to weeks from prolonged rainfall or snowmelt, often lasting a long time. Flash floods occur rapidly with little warning, triggered by intense local storms or sudden dam failures, and can be extremely hazardous.
What are some methods used for flood control, and what can go wrong?
Flood control includes levees/floodwalls to contain high water and dams to store and release floodwaters gradually. These can fail if overwhelmed, potentially leading to catastrophic flooding (e.g., Hurricane Katrina levee breaches).
How do geologists and engineers evaluate flood hazards?
They use recurrence intervals (like the 100-year flood with a 1% annual probability), flood-hazard maps to identify at-risk areas, and regulations/land-use planning to discourage construction on floodplains.
