The term describing the total volume of solid material transported by a stream past a specific point over a given time is the Total Sediment Load, often used interchangeably with Sediment Discharge. This load includes all particulate matter, from microscopic clay to large boulders, that the flowing water moves downstream. Hydrologists use the magnitude of this load to measure a river’s erosional power and channel stability. Understanding the total sediment load is fundamental for managing water resources, predicting channel changes, and assessing environmental impacts. This volume consists of distinct fractions, each transported by a different physical mechanism.
The Components of Total Sediment Load
The total sediment load is separated into components based on particle size and transport mechanism. The coarsest material travels as the bed load, consisting of particles too dense to be lifted high into the water column. This fraction moves along the streambed by rolling, sliding, or through saltation, a bouncing motion where grains make intermittent contact with the channel floor. Bed load particles are typically sand, gravel, or cobbles, moving primarily during high flow when the water’s tractive force is greatest.
The suspended load is the material held up within the water column by flow turbulence. These particles are generally finer than the bed load, consisting mainly of fine sand, silt, and clay. The amount of suspended sediment relates directly to water velocity and turbulence intensity, which overcomes the particles’ tendency to settle. In many streams, the suspended load accounts for the largest proportion of the total sediment volume transported.
The wash load is the finest fraction of the suspended material, consisting almost entirely of clay and fine silt. This sediment remains in near-permanent suspension, carried at virtually the same velocity as the water itself. Wash load particles are not found in significant quantities in the streambed, meaning they are sourced from watershed runoff and slopes rather than channel erosion. The volume of wash load is determined by the availability of fine sediment in the entire drainage basin, not by the stream’s specific transport capacity.
Controls on Stream Sediment Transport
The magnitude of the total sediment load is dynamic and governed by physical and environmental factors. The most direct influence is the stream’s discharge and velocity, as the ability of water to entrain and move sediment relates exponentially to its flow speed. A small increase in velocity, such as during a storm, can lead to a disproportionately large increase in the amount and size of sediment carried. This relationship is quantified by stream power, which represents the rate at which the flowing water expends energy.
Stream gradient and channel geometry also control the water’s transport capacity. A steeper gradient leads to higher flow velocities, increasing the shear stress exerted on the streambed. The channel shape influences turbulence patterns; narrow or irregular channels generate localized turbulence that helps sustain sediment in suspension. Changes in the channel’s cross-sectional area, such as during a flood, also affect flow velocity and resulting sediment transport.
External factors, specifically sediment availability and land use within the watershed, determine the source material entering the stream system. The underlying geology and soil type dictate the natural size and quantity of erodible material. Human activities, such as deforestation, agriculture, and construction, significantly increase the supply of fine sediment available for transport. When the sediment supply is high, the stream’s transport capacity can become the limiting factor, affecting the overall measured load.
Methods for Quantifying Sediment Load
Hydrologists use specialized equipment and techniques to quantify the total sediment load components in the field. Suspended sediment is commonly measured using depth-integrating samplers, such as the US D-49 or the US DH-48. These devices are lowered and raised at a uniform rate, collecting a continuous, discharge-weighted sample of the water-sediment mixture across the vertical profile. The collected sample is then analyzed in a laboratory to determine the sediment concentration by weight, typically expressed in milligrams per liter.
Measuring the bed load is more challenging because the material moves along the bottom in a turbulent and intermittent manner. Specialized tools like the Helley-Smith sampler are used to physically capture the rolling and saltating particles. This sampler features a square entrance nozzle leading to a fine-mesh bag that traps the sediment moving near the bed. Measurements are taken at multiple points across the stream’s width and then extrapolated to estimate the total bed load transport rate for the entire cross-section.
For situations where direct measurement is impractical or incomplete, the total load can be estimated using various modeling and indirect techniques. Empirical formulas, such as the Meyer-Peter Müller equation, are widely used to predict the bed load transport rate based on hydraulic parameters like stream power and shear stress. This formula was developed from laboratory experiments and is most applicable for coarse sand and gravel beds. These models allow for the prediction of sediment movement under a range of flow conditions.