- A geomorphologist studying a wide river valley observes several oxbow lakes, abandoned meander scars, and natural levees composed of fine sand and silt. These features collectively provide the strongest evidence for which conclusion about the river's long-term behavior?
Answer: The river has undergone significant lateral migration and periodic overbank flooding.
Oxbow lakes and meander scars are direct evidence of the lateral (side-to-side) migration of a meandering river channel over time. Natural levees are ridges of sediment deposited along the channel banks during floods when the river overtops its banks. The presence of all these features indicates a mature river system that actively shifts its course across a floodplain and experiences regular flood events that are part of its natural cycle.
- A geologist studies two rivers. River A drains a semi-arid region with a highly variable discharge and an abundant supply of coarse sediment. River B flows through a humid, temperate region with a stable discharge and cohesive, silty banks. Which channel patterns are most likely characteristic of these two rivers?
Answer: River A: Braided; River B: Meandering
Braided channels are characterized by multiple, intertwining channels separated by bars. They form in environments with high sediment loads (especially coarse sediment), variable discharge, and easily eroded banks—all characteristics of River A. Meandering channels are single, sinuous channels that form in rivers with more stable flows, lower gradients, and more cohesive bank material that supports a single deep channel—all characteristics of River B.
- A drop of rain falls at a location with coordinates 44° N, 110° W, which is within Yellowstone National Park, Wyoming, just west of the Continental Divide. Assuming it flows into a stream and is not evaporated, into which major body of water will it ultimately drain?
Answer: The Pacific Ocean
The Continental Divide of the Americas separates watersheds that drain to the Pacific Ocean from those that drain to the Atlantic and Arctic Oceans (including the Gulf of Mexico and Hudson Bay). The location 44° N, 110° W is in the headwaters of the Snake River system. The Snake River is a major tributary of the Columbia River, which flows into the Pacific Ocean. Drainage to the Gulf of Mexico (via the Missouri/Mississippi) would originate east of the divide. The Great Salt Lake is in its own endorheic (closed) basin, and Hudson Bay drainage is further north and east.
- A steep, sediment-laden mountain stream emerges from a confined canyon and spreads out onto a broad, flat desert plain. This sudden decrease in gradient and confinement will cause the stream to deposit much of its sediment, forming which type of landform?
Answer: An alluvial fan
An alluvial fan is a fan- or cone-shaped deposit of sediment built up by streams. They are common in arid and semi-arid regions where a stream flowing out of a steep, narrow canyon experiences a rapid decrease in slope and flow confinement. This causes the stream to lose energy and deposit its sediment load. A delta forms where a river enters a standing body of water, a point bar forms on the inside of a meander, and a natural levee forms along the banks during a flood.
- The chart shows the peak annual discharge for a river over a 50-year period. Based on this historical record, what is the best estimate for the discharge of a flood with a 25-year recurrence interval?
Answer: 2600 m³/s
A flood with a 25-year recurrence interval is a flood that is statistically expected to be equaled or exceeded, on average, once every 25 years. In a 50-year record of annual peak flows, this corresponds to the flood that was equaled or exceeded 50/25 = 2 times. Therefore, we should look for the second-highest flood in the dataset. The highest is 3000 m³/s, and the second-highest is 2600 m³/s.
- A geologist examines a region where the bedrock consists of a series of parallel, folded ridges and valleys composed of alternating layers of resistant sandstone and easily eroded shale. Which type of drainage pattern would be most expected to develop in this landscape?
Answer: Trellis
A trellis drainage pattern is characteristic of landscapes with folded or tilted sedimentary rocks of varying resistance. The primary streams tend to follow the axes of the valleys eroded into the weaker rock (shale), while short tributaries flow down the sides of the resistant ridges (sandstone), joining the main stream at nearly right angles. This creates a pattern resembling a garden trellis.
- A dam is constructed on a major river that historically transported a large volume of sand and gravel. What is the most likely long-term effect on the river channel immediately downstream of the dam?
Answer: Incision of the channel and coarsening of the remaining bed material.
When you encounter questions about dam construction and river systems, focus on how dams disrupt the natural balance between sediment supply and the river's energy to transport material.
Before dam construction, rivers maintain equilibrium between their capacity to carry sediment and the sediment load supplied from upstream. When a dam blocks this system, it traps the sand and gravel that would normally flow downstream, creating "hungry water" – clear water with excess energy that's no longer being used to transport its usual sediment load.
This excess energy gets redirected into eroding the riverbed and banks immediately downstream of the dam. As the channel cuts deeper (incision), the finer particles like silt and clay are preferentially removed, leaving behind coarser materials like gravel and rocks. This process is called armoring, where the bed becomes dominated by material too large for the reduced sediment load to move.
Option A correctly identifies both incision and coarsening – the two key consequences of sediment starvation below dams. Option B is wrong because aggradation (sediment buildup) occurs upstream of dams, not downstream, and the downstream area loses fine material rather than accumulating it. Option C incorrectly suggests increased meandering, but channel incision actually reduces the river's ability to migrate laterally and form meanders. Option D focuses on temperature changes, which may occur but ignores the primary geomorphic impacts that are central to this type of question.
Remember: dams create sediment-starved water downstream that compensates by eroding the channel bed and removing fine materials.
- A particular location on a river experienced a flood in 2015 that was statistically determined to be a '100-year flood.' Which statement most accurately describes the likelihood of a flood of the same magnitude or greater occurring in 2016?
Answer: The probability is 1%, as the occurrence of the flood in the previous year does not change its statistical probability for any given year.
The term '100-year flood' refers to a flood event that has a 1 in 100, or 1%, chance of being equaled or exceeded in any given year. Each year is treated as an independent statistical event. The occurrence of a 100-year flood in one year has no bearing on the probability of it occurring the next year. Therefore, the probability remains 1%.
- The diagram shows a bend in a meandering river. At which location is the water velocity highest, causing the greatest potential for bank erosion?
Answer: Point A
In a meander bend, centrifugal force directs the fastest-moving water (the thalweg) toward the outer bank. This high-velocity flow at Point A, the cut bank, has the most erosive power and actively undercuts the bank. In contrast, the water at Point B on the inner bank (point bar) is slower, leading to deposition. Velocity at Point C, in the center of a straight reach, is typically less than the maximum velocity found at the outer bend.
- Using the Strahler method for stream ordering, what is the stream order at point Z in the drainage network shown?
Answer: 3rd order
In the Strahler method: 1) Headwater streams are 1st order. 2) When two streams of the same order join, the resulting stream's order increases by one. 3) When two streams of different orders join, the resulting stream retains the higher of the two orders. In the diagram, two 1st order streams join to form a 2nd order stream. This 2nd order stream is then joined by another 1st order stream, and the channel remains 2nd order. At point Z, two 2nd order streams join, so the resulting stream becomes a 3rd order stream (2 + 1 = 3).
- Drainage density is the total length of all stream channels in a basin divided by the area of that basin. Consider two basins of equal area: Basin X consists of impermeable clay soils with sparse vegetation, while Basin Y has porous sandy soils and is covered by a dense forest. Which basin is expected to have a higher drainage density, and why?
Answer: Basin X, because low infiltration rates lead to greater surface runoff, which carves more channels.
When you encounter drainage density questions, focus on the relationship between surface water flow and channel formation. Drainage density increases when more water flows over the surface rather than soaking into the ground, because surface runoff is what carves and maintains stream channels.
Basin X's impermeable clay soils prevent water from infiltrating into the ground, forcing most precipitation to flow as surface runoff. This abundant surface water creates and maintains numerous stream channels. The sparse vegetation provides little protection against erosion and doesn't significantly increase infiltration rates. Additionally, with minimal plant roots to hold soil together, the surface is more susceptible to channel formation by flowing water.
Let's examine why the other options miss the mark. Option B incorrectly suggests that forest root systems create drainage channels - in reality, roots help water infiltrate into soil and actually reduce surface runoff, leading to fewer stream channels. Option C makes the fundamental error of assuming drainage density depends only on basin area, when it's actually determined by the balance between surface runoff and infiltration. Option D presents a nonsensical explanation about clay weight creating stream space, which has no basis in hydrology - soil compression doesn't create drainage networks.
The correct answer is A because low infiltration in clay soils maximizes surface runoff, which carves more channels per unit area.
Study tip: Remember that drainage density is inversely related to infiltration capacity. High infiltration (sandy soils, dense vegetation) means low drainage density, while low infiltration (clay soils, sparse vegetation) means high drainage density.
- A wildfire burns a significant portion of a steep, mountainous drainage basin, removing most of the vegetation and litter layer. What are the most likely immediate effects on the basin's main stream during the first major rainstorm after the fire?
Answer: Increased peak discharge and increased sediment load, due to reduced infiltration and loss of soil stability.
The removal of vegetation and the organic litter layer by fire has two main effects. First, it reduces the soil's ability to absorb water (infiltration), leading to more surface runoff. This causes a higher peak discharge and a shorter lag time. Second, the loss of plant roots and ground cover destabilizes the soil, making it highly susceptible to erosion by rain splash and runoff. This results in a massive increase in the sediment load of the stream.
- The graph shows a rainfall event (hyetograph) and the resulting streamflow response (hydrograph) for a watershed. Based on an analysis of the two data series, what is the approximate basin lag time?
Answer: 4 hours
Basin lag time is defined as the time difference between the center of mass of the rainfall event and the peak of the resulting hydrograph. A common approximation is the time from the peak rainfall intensity to the peak stream discharge. In the graph, the peak rainfall intensity occurs at hour 2. The peak stream discharge occurs at hour 6. The lag time is the difference between these two peaks: 6 hours - 2 hours = 4 hours.
- The diagrams show two distinct drainage patterns. Which option correctly pairs each pattern with the geological setting that most likely controls its formation?
Answer: Pattern 1: Uniformly resistant, flat-lying rocks; Pattern 2: Folded rock layers of varying resistance.
Pattern 1 is a dendritic pattern, which resembles the branching of a tree. It develops on surfaces where the underlying bedrock has uniform resistance to erosion, such as flat-lying sedimentary rocks or massive igneous intrusions. Pattern 2 is a trellis pattern, characterized by a system of parallel main streams with short tributaries joining at right angles. This pattern is controlled by underlying geologic structure, typically developing on parallel bands of tilted or folded sedimentary rocks with alternating levels of resistance.
- Due to long-term climate change, global sea level rises by several meters over a century. How would this change in ultimate base level most likely affect the lower reaches of a major river system that was previously in equilibrium?
Answer: The river's gradient will decrease, leading to a reduction in velocity and widespread sediment deposition (aggradation).
A rise in sea level raises the ultimate base level of a river. This reduces the river's overall gradient, particularly in its lower reaches near the coast. According to the principles of stream dynamics, a decrease in gradient leads to a decrease in water velocity and, consequently, a reduction in the river's capacity to transport sediment. This causes the river to deposit its load, a process known as aggradation, building up the riverbed and floodplain.
- A city builds artificial levees to confine a river to its channel and prevent flooding of adjacent neighborhoods. What is a common, unintended consequence of this action for the river system during a major flood?
Answer: It increases the water level and velocity within the channel, potentially worsening flooding downstream.
Artificial levees prevent floodwater from spreading out over the wide floodplain. By constraining the same volume of water into a narrower cross-section, the levees force the water level (stage) to rise higher and the velocity to increase. This constricted, high-energy flow is then passed downstream, where it can lead to more severe flooding in unprotected areas.
- A severe thunderstorm drops a uniform amount of rain across a small, undeveloped drainage basin. The resulting stream discharge is shown by the solid line in the hydrograph. Years later, the basin is heavily urbanized with extensive pavement and storm sewers. If an identical storm occurs, which dashed line best represents the new hydrograph?
Answer: Curve A
Urbanization increases the amount of impervious surfaces (pavement, rooftops), which reduces infiltration and accelerates runoff into streams via storm sewers. This causes two primary effects on the hydrograph: 1) the lag time (the time between peak rainfall and peak discharge) decreases, and 2) the peak discharge increases because more water arrives at the channel over a shorter period. Curve A correctly shows a higher, steeper peak that arrives sooner than the original hydrograph.
