Question

If you collect a jar of water from a stream, what part of its load will settle to the bottom of the jar? What portion will remain in the water indefinitely? What part of the stream's load would probably not be represented in your sample?

Short answer

Heavy particles settle at the bottom, fine particles remain suspended, and dissolved minerals aren't visible. Large bed load particles aren't collected.

Step-by-step solution

Understanding the Load Types

A stream's load refers to the materials it transports, which are typically categorized into three types: bed load, suspended load, and dissolved load. Bed load consists of larger particles like sand, gravel, and boulders that roll or slide along the bottom. Suspended load includes smaller particles like silt and clay that remain suspended in the water. Dissolved load comprises materials that are completely dissolved, like salts and minerals.

Identifying Settling Particles

When a jar of stream water is left to settle, the particles that are too heavy to remain suspended will eventually settle at the bottom. These generally include parts of the bed load and larger portions of the suspended load like sand grains. Thus, the settled material is likely composed of the heaviest particles, primarily sand and some silt, depending on the flow level of the stream.

Determining Indefinitely Suspended Particles

The smallest, lightest particles, such as fine silt and clay, remain in suspension indefinitely because they are too small for gravitational settling to cause them to fall out of the water column. These particles reflect the finer part of the suspended load that stays mixed with the water.

Recognizing the Absent Loads

The dissolved load, which includes minerals and salts dissolved in the water, does not form a solid matter that can be seen separately in the jar sample. Thus, these dissolved materials are present in the jar but invisible as they do not form sediment at the bottom. Additionally, the largest materials from the bed load, such as rocks and boulders, are not collected in a jar, since they do not travel with the main water body but instead move by rolling along the stream bed.

Key concepts

Bed Load

When we talk about stream loads, the bed load is an essential component that travels along the bottom of a stream. This consists of larger particles like gravel, sand, and boulders that are too heavy to be carried in the water column. Due to their weight, they roll, slide, or hop along the bed of the river. If you scoop up a jar of water from a stream, the bed load is unlikely to be represented in your sample. This is because larger materials usually require fast-moving water to move. They consist mostly of larger particles that settle quickly and are harder to collect with stationary water.
In a typical jar sample from a stream, you'd most likely not find rocks or large debris at the bottom. However, if the stream is agitated or particularly energetic, smaller elements of the bed load, like sand, may settle in your jar if given time. Understanding this helps us know what makes up the visible materials at the bottom when we gather a jar sample.

Suspended Load

The suspended load is made up of fine particles such as silt and clay that remain floating within the water, giving streams a cloudy or murky appearance. Unlike bed load materials, suspended particles are small enough to be lifted and carried by the most average flow rates. This feature is why they are present prominently in any given water sample.
When a jar of stream water is collected, over time, some particles from the suspended load may settle to the bottom. However, the finest sediments, like clay, can remain suspended for indefinite periods, driven by water currents or motion within the jar. These particles reflect the significant part of the stream's material load that is briefly visible but primarily remains intermixed rather than settling at the jar's bottom.

• Contains fine particles like silt and clay
• Gives streams their murky appearance
• Some particles may settle in a jar over time

Dissolved Load

Dissolved load refers to the components of a stream that are completely dissolved in the water. These include minerals like calcium, chlorine, and bicarbonates, which invisible to the naked eye in a jar. This load results from water dissolving different minerals from rocks and soil as it flows. Unlike the bed and suspended loads, dissolved materials remain permanently mixed with the water, and their presence does not lead to any sediment deposit in a water sample.
Essentially, when you collect a jar of stream water, the dissolved load constitutes what is "hidden" within the liquid: minerals that cannot be filtered out just by letting the water sit still. Analyzing the dissolved load requires chemical testing as opposed to visual observation. Recognizing these components allows us to appreciate the chemical journey water undertakes as it traverses landscapes.

Sediment Settling

Sediment settling is a process that describes how particles, once suspended in water, begin to fall to the bottom due to gravity. This phenomenon occurs when the kinetic energy of flowing water is insufficient to keep particles moving, causing them to eventually deposit as sediment. In a stream jar sample, sediment settling will manifest over time as the heavier particles begin to form a layer at the bottom.
This process primarily affects the heavier portions of the suspended load, like sand grains, which will settle first, leaving a clearer layer of water above. Analyzing the layers formed by sediment settling provides insights into the particle size and composition of materials within the water. This understanding is essential for scientists studying erosion patterns, the rate of flow, or even sediment transportation within streams.

• Heavier particles settle faster
• Creates visible layers in a jar
• Influenced by particle size and water flow

Water Sample Analysis

Water sample analysis is a crucial step in understanding the different loads present in a stream. When we analyze a water sample, we can determine what materials are present and in what form. This involves both physical observation, like noting what settles at the bottom of a jar, and chemical analysis, for elements dissolved invisibly in the water.
The analysis process can provide insight into factors like pollution level, sediment composition, and environmental health. For instance, unusually high sedimentation might point toward increased erosion upstream. Similarly, a detailed chemical analysis might reveal elevated concentrations of dissolved minerals potentially indicative of recent geological or human activity.
To accurately assess a stream's load, detailed steps include:

• Settling observations for physical particles
• Chemical tests for dissolved materials
• Mathematical calculations to quantify particulate distribution
• Environmental studies to deduce broader implications

These efforts contribute to a comprehensive understanding of how streams transform and transport their load, shaping environments over time.