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 caused by heavy rain increased streamflow velocity and turbulence by altering the stream's gradient and channel shape.

Step-by-step solution

Understanding Mass Wasting

Mass wasting refers to the downhill movement of rock and soil material due to gravity. Understanding this is crucial to envision how it affects a stream's flow.

Scenario Setup

Let's imagine a steep hillside located near a stream. Over time, heavy rainfall saturates the soil on the hillside, increasing its weight and reducing its cohesion. Additionally, the tree roots that initially provided stability begin to rot due to excess water.

Triggering the Mass Wasting Event

The mass wasting event is triggered when an intense storm delivers more rain, pushing the soil beyond its saturated limit. This causes a landslide, where a large volume of earth material moves rapidly downhill into the stream below.

Impact on Streamflow

The landslide suddenly deposits a large amount of rock and soil into the stream, obstructing its path. This increases the stream's gradient and modifies its channel shape, causing an increase in the water velocity and turbulence downstream.

Changes in Discharge and Roughness

The mass wasting adds debris to the stream, altering its discharge and channel roughness. Initially, the obstruction could reduce flow, but as water builds up and breaches, the flow rate may increase sharply, carrying more sediments due to increased channel roughness.

Key concepts

Understanding Streamflow

Streamflow refers to the movement of water within a stream channel and is influenced by various factors, including the amount of water entering the stream and the characteristics of the stream bed itself. These factors determine how fast the stream water moves and how much sediment it carries.
Streamflow plays a critical role in shaping the land, as it erodes, deposits, and transports sediment from one location to another.

• The velocity of water in the stream is key to understanding how erosion occurs. Faster-moving water can carry larger particles and more debris.
• Volume of water or discharge is a crucial factor in determining the power of streamflow.

Understanding these dynamics helps explain how changes in the environment, like mass wasting events, can lead to significant modifications in stream behavior.

What Is a Landslide and How It Occurs

A landslide occurs when a large mass of earth—such as soil, rocks, and other debris—moves down a slope under the influence of gravity. Landslides can happen suddenly and with little warning, especially when external factors, such as heavy rainfall, contribute to slope instability.
Several factors can trigger a landslide:

• Prolonged rainfall saturating the soil, making it heavier and decreasing its stability.
• Loss of vegetation roots which help stabilize the soil.
• Rapidly occurring events like an intense storm that provide sudden changes in water pressure, leading to abrupt collapse of earth materials.

A landslide can drastically change the nearby ecosystem, particularly affecting any adjacent streams by depositing large amounts of material into them, thus altering their flow.

How Channel Roughness Affects Streams

Channel roughness refers to the texture and irregularities present on the stream bed and banks. It is usually caused by features like rocks, vegetation, and man-made structures. Roughness plays a vital role in determining how water flows through a stream.
Here's how channel roughness impacts streams:

• Increased roughness slows down the water flow, as it has to navigate around obstacles.
• It can increase turbulence, leading to greater mixing of water, which affects sediment transport.
• During a mass wasting event, added debris increases channel roughness, causing initial blockage and potential changes in water velocity over time.

The combination of increased gradient and roughness after a landslide can dramatically change a stream's dynamics, potentially leading to increased erosion downstream.

The Role of Discharge in Streamflow

Discharge is the volume of water flowing through a stream channel at a given time and is expressed in cubic meters per second ( q m/s). It depends on two main factors: the cross-sectional area of the stream and the velocity of the water.
High discharge rates can lead to faster-moving water, capable of transporting more sediment and debris. This can result in:

• Increased erosion of the streambanks and bed.
• Greater capacity to carry sediment further downstream.
• Potential flooding in areas where the stream cannot handle the increased discharge.

A mass wasting event can initially reduce discharge but eventually increases it once the blockage is overcome, leading to a surge effect that further changes the stream’s characteristics.

Understanding Stream Gradient

The stream gradient is the slope or steepness of the stream channel and is a critical factor in determining streamflow velocity. A steeper gradient generally results in faster-moving water, which can lead to more aggressive erosion and sediment transport.
Events like landslides can change the gradient:

• The blocking of a stream by landslide debris might temporarily increase the upstream gradient.
• Once water breaches the blockage, it may cascade with increased energy, altering downstream gradients.
• Changes in gradient can make streams either more erosive or prone to deposition.

Through understanding stream gradient, observers can anticipate the long-term impacts of natural events and how they might reshape landscapes.