What Are The Three Types Of Loads Carried By Streams

What Are the Three Types of Loads Carried by Streams?

Streams, rivers, and other flowing bodies of water are powerful agents of erosion and transportation. They carry a significant amount of material, collectively known as stream load, from their source to their mouth. This load is categorized into three main types: dissolved load, suspended load, and bed load. Understanding these different types of loads is crucial for comprehending fluvial geomorphology, sediment transport dynamics, and the overall impact of rivers on the landscape. This article will delve deep into each of these types, exploring their characteristics, transport mechanisms, and significance in shaping river systems.

1. Dissolved Load: The Invisible Traveler

The dissolved load comprises materials that are completely dissolved in the water. These are typically soluble ions, such as calcium, sodium, magnesium, bicarbonate, sulfate, and chloride. These ions originate from the weathering and dissolution of rocks and minerals within the drainage basin. Rainwater, slightly acidic due to dissolved carbon dioxide, reacts with minerals like limestone and granite, breaking them down and releasing these ions into solution.

Sources and Processes Contributing to Dissolved Load:

• Chemical Weathering: This is the primary source. The chemical breakdown of minerals, especially those susceptible to dissolution, releases ions directly into the water. This is particularly significant in areas with carbonate rocks (like limestone) and evaporite deposits (like salt flats).

• Solution: The process where soluble minerals are dissolved and transported in solution. This is highly dependent on the water's chemistry (pH, temperature, presence of dissolved gases) and the mineralogy of the rocks within the watershed.

• Ground Water Contribution: Groundwater often carries a significant dissolved load, as it percolates through the subsurface, dissolving minerals along the way. Rivers gain a substantial portion of their dissolved load from groundwater inflow.

Characteristics of Dissolved Load:

• Invisible: As the name suggests, it is invisible to the naked eye. Its presence is only detectable through chemical analysis.

• Constant Transport: Unlike suspended and bed load, dissolved load is continuously transported; it’s not deposited and re-eroded as water velocity fluctuates.

• High Concentration in Certain Environments: The concentration of dissolved load varies greatly depending on the geology of the catchment area. Areas with carbonate rocks typically have higher dissolved loads compared to areas with resistant igneous or metamorphic rocks.

Significance of Dissolved Load:

• Water Chemistry: It significantly influences the chemical composition of rivers and ultimately oceans.

• Nutrient Cycling: Many essential nutrients for aquatic life are transported as part of the dissolved load.

• Sediment Budget: While not visible sediment, it is a key component of the total sediment budget of a river system.

2. Suspended Load: The Floating Particles

The suspended load consists of fine-grained sediment particles, such as silt and clay, that are held aloft by the water's turbulence. These particles remain in suspension for extended periods, even with relatively low flow velocities. The size and density of the particles, along with the water's velocity and turbulence, determine the amount of suspended load a stream can carry.

Factors Affecting Suspended Load Transport:

• Water Velocity: Higher velocities keep more particles in suspension.

• Turbulence: Turbulence prevents particles from settling. Features like meanders and rapids increase turbulence.

• Particle Size and Density: Fine-grained, low-density particles (silt and clay) are more easily suspended than coarser, denser particles (sand and gravel).

• Discharge: Higher discharge (volume of water flowing past a point per unit of time) increases the capacity to carry suspended load.

Characteristics of Suspended Load:

• Visible Turbidity: Suspended load makes water appear muddy or cloudy. The higher the concentration of suspended sediment, the more turbid the water.

• Variable Transport: While mostly suspended, particles may settle out during periods of low flow or in quieter stretches of the river. They are readily resuspended when flow increases.

• Significant Volume: Suspended load often comprises the largest portion of the total sediment load, particularly in larger rivers.

Significance of Suspended Load:

• Water Quality: High concentrations of suspended sediment can impair water quality, reducing light penetration and affecting aquatic life.

• River Morphology: Suspended sediment is a significant factor in shaping river channels and floodplains through deposition.

• Coastal Processes: Rivers carry immense amounts of suspended load to the coast, contributing to delta formation and coastal erosion/accretion patterns.

3. Bed Load: The Rolling and Sliding Material

The bed load consists of coarser sediment particles, such as sand, gravel, and cobbles, that are transported along the streambed. These particles are too heavy to be kept in suspension by the water's turbulence and move through a combination of rolling, sliding, and saltation (a series of short hops).

Mechanisms of Bed Load Transport:

• Traction: Larger particles are moved by the force of the water flowing directly over them. This involves rolling or sliding along the streambed.

• Saltation: Smaller particles are lifted briefly into the water column by turbulent eddies, then fall back to the bed, moving in a series of jumps.

• Impact: Moving particles can strike other particles, causing them to move as well.

Factors Influencing Bed Load Transport:

• Water Velocity: A critical velocity is needed to initiate bed load movement. Higher velocities lead to increased transport rates.

• Particle Size and Shape: Larger, heavier particles require higher velocities for transport. Particle shape also plays a role; more rounded particles are easier to move than angular ones.

• Stream Slope: Steeper slopes generally result in higher velocities and increased bed load transport.

• Bed Roughness: A rougher streambed provides more resistance to flow and reduces the efficiency of bed load transport.

Characteristics of Bed Load:

• Visible Movement: Bed load movement is readily observable during periods of high flow.

• Intermittent Transport: Particles are not constantly in motion. They move intermittently, depending on flow conditions.

• Localized Movement: Bed load transport is concentrated near the streambed.

Significance of Bed Load:

• Channel Morphology: Bed load transport shapes the river channel through erosion and deposition. It's crucial for understanding channel changes, meandering, and braiding patterns.

• Habitat Creation: The movement and deposition of bed load materials create varied habitats for aquatic organisms.

• River Engineering: Understanding bed load transport is vital for river management and engineering projects.

Interplay Between the Three Loads

It's essential to remember that these three types of loads are not mutually exclusive. They often interact and influence each other. For instance, the erosion of the streambed by bed load movement can release finer particles into the suspended load. Similarly, changes in discharge can affect all three types of loads simultaneously, triggering deposition or erosion events.

Conclusion: Understanding Stream Loads for a Holistic Perspective

The dissolved, suspended, and bed loads of a stream represent a dynamic system reflecting the complex interplay between water, sediment, and the landscape. By studying these components individually and understanding their interactions, we can gain a much more complete understanding of fluvial processes, predict river behavior, manage water resources effectively, and mitigate the impacts of environmental changes. A holistic approach considering all three types of loads is paramount for informed decision-making in areas ranging from watershed management and flood control to assessing the ecological health of river systems.