Question

Streamflow is affected by several variables, including discharge, gradient, and channel roughness, size, and shape. Develop a scenario in which a mass wasting event influences a stream's flow. Explain what led up to, or triggered, the event and describe how the mass wasting process influenced the stream's flow.

Short answer

A landslide triggered by heavy rain and an earthquake blocked the stream, altering flow and causing potential upstream flooding.

Step-by-step solution

Understanding Mass Wasting

Mass wasting is the movement of soil and rock downslope under the influence of gravity. It can be triggered by factors such as heavy rainfall, earthquakes, volcanic activity, or human interference, like mining or construction. It is important to understand these factors as they can critically alter landscapes and affect streams and rivers in their path.

Scenario Development

Consider a steep hillside located above a river. Following several days of heavy rain, the soil becomes saturated and unstable. An earthquake occurs, adding stress and causing the slope to fail. This leads to a landslide, where a large mass of soil and rock rapidly slides down the hill into the river valley below.

Initiating Factors

In this scenario, the mass wasting event is triggered by the combination of factors: heavy rainfall leading to soil saturation, and an earthquake providing the sudden force needed to initiate the landslide. The presence of loose soil and a steep slope margin increases the susceptibility to such an event.

Impact on Stream Flow

When the landslide material enters the river, it partially or completely blocks the stream flow, creating a temporary natural dam. The influx of sediment can increase the river's sediment load, altering the gradient and increasing channel roughness. This might temporarily raise water levels upstream, potentially flooding areas, while downstream flow may be reduced or completely obstructed until the dam is breached.

Long-term Effects

Over time, the natural dam might be eroded by the water, collapsing and releasing a surge of water and debris downstream. This can lead to increased erosion downstream as the current works to reestablish equilibrium in the sediment load and flow. The event may have lasting effects on the river's shape due to sediment deposition in new areas, altering habitats and possibly the local landscape.

Key concepts

Streamflow

Streamflow refers to the movement of water through a river channel. It is a crucial part of the water cycle and can be influenced by several factors such as discharge, which is the volume of water flowing through the channel at any given time, gradients or slopes, and channel attributes like roughness, shape, and size. All these elements interact to dictate how smoothly or turbulently water moves downstream.

• Discharge: More water in a river increases flow velocity and can influence erosion and sediment transport.
• Gradient: A steeper gradient usually results in faster flowing water, enhancing its erosive power but it becomes dangerous during floods.
• Channel Roughness: Rocks, debris, and plant life in a river can slow down water flow and increase turbulence.

When a mass wasting event occurs, such as a landslide, it can drastically alter these factors by introducing additional materials into a river channel, potentially blocking the flow and forming a natural dam. This change in the characteristics of streamflow can have profound effects, such as flooding upstream areas or reducing water availability downstream.

Landslides

Landslides are common types of mass wasting events where Earth materials rapidly move down a slope. They can occur with varying speeds and impacts, largely influenced by gravity and external factors. A landslide might be triggered by:

• Heavy Rainfall: Water adds weight and reduces the friction that holds materials together, making slopes unstable.
• Earthquakes: Sudden movements can dislodge materials on slopes.
• Human Activities: Deforestation, construction, and mining can destabilize slopes.

In our scenario, a landslide was triggered by the saturation of soils from prolonged rain and the added force of an earthquake. When such an event happens, it can modify streamflow by depositing rock and soil directly into the riverbed, potentially creating a blockage or increasing sediment load. This interaction highlights the dynamic relationship between landforms and waterways.

Sediment Load

Sediment load refers to the materials that rivers carry as they flow. This can include clay, silt, sand, gravel, and other particles like organic matter.

• Sources of Sediment: Weathering and erosion of rocks and soil are primary sources.
• Impact of Increased Load: When a river's sediment load increases, it can change the river's flow characteristics. For example, increased load can cause the river to slow and deposit excess sediment, altering channel shape and habitat.

During a mass wasting event, like a landslide, the sediment load in a river can drastically increase. The sudden influx of material changes the gradient and friction in the river channel, sometimes causing it to widen or alter its course. This can have long-term effects on the environment, potentially affecting aquatic habitats and even altering the surrounding landscape over time.

Natural Dam

A natural dam is a barrier formed from landslide debris or other natural materials, blocking a river. This can lead to the formation of temporary lakes and create a wide array of ecological and environmental impacts.

• Formation: When landslide debris falls into a river valley, it may pile up to obstruct water flow completely or partially.
• Immediate Effects: It can raise water levels upstream, increasing the risk of flooding and impacting ecosystems.
• Long-term Impacts: Over time, natural dams are often breached by water erosion, leading to sudden floods and downstream property damage.

In the scenario of a mass wasting event influencing a river, a natural dam creates a short-term barrier that disrupts normal streamflow, creates backwater effects upstream, and decreases flow downstream until it is broken down. This process illustrates the powerful impact of mass wasting events on natural waterways and demonstrates the interconnectedness of land and water processes.