Geocentric Science

Once rain or other forms of precipitation hits the Earth’s surface, it can either infiltrate (seep into the ground), evaporate, undergo transpiration (water vapor released by plants), or become runoff. All of those potential paths play an important role in the water cycle.

Image from Wikipedia

Runoff is precipitation that does not infiltrate or evaporate. Factors that determine how much water flows across the surface include:

• The amount of water present
• The moisture of the soil
• The soil texture
• The slope of the land
• The vegetation

Water can flow as a thin sheet (known as overland flow) across the surface and can cause sheet erosion.

Vegetation helps to slow and reduce sheet erosion. The plant roots help to keep the soil in place. This is especially important on steep slopes, where there is little time for the water to be absorbed into the ground. If there is a lack of vegetation, a gully can form, and if erosion continues, a gulch can form.

A gully is a small ditch or funnel created by overland flow as the runoff moves downhill.

A gulch a deep V-shaped valley.

Eventually, the runoff will reach a stream or a river. The stream’s velocity will determine how the stream will flow and the size of sediment that is being transported. The diagram Relationship of Transported Particle Size to Water Velocity on page 6 of the Earth Science Reference Tables, shows how stream velocity affects the carrying capacity; as the velocity increases, the stream is able to transport larger sediment. Additionally, as the velocity decreases, the largest sediment will be deposited first.

1. How does water flow in a stream?

   Answer:

   A. Laminar Flow: Water particles flow in a straight line parallel to the stream channel (defined stream path). There is very little mixing of water particles.

   B. Turbulent Flow: Water moves in a random fashion; water swirls, whirlpools are created, and water particles are mixed.

2. How does a stream transport sediment?

   Answer: A river transports its sediment, also known as “load”, in several ways, see below:

   A. In Solution: Also referred to as the dissolved load. Water dissolves the bedrock over time, dissolving sediments and minerals into the water, which is transported in this load. The velocity does not effect the stream’s ability to carry particles within the dissolved load. Once the minerals are in solution, they remain in solution until they are precipitated out.

   B. In Suspension: Also referred to as the suspended load. This load consists of the small sediment particles (fine sand, silt, and clay). The stream’s velocity does effect the amount of material carried in this load.

   C. Bed Load: Larger sediments are moved along the bottom of the stream channel by rolling, sliding, and saltation (jumping or bouncing). The bed load is the main contributor to downcutting (the downward weathering and erosion of a stream into its channel).

3. How does a stream or river erode?

   Answer: The water velocity will determine the size of the sediment transported (refer to the diagram Relationship of Transported Particle Size to Water Velocity on page 6 of the Earth Science Reference Tables). The diagram indicates that as water velocity increases, the water is able to transport larger sediment. For example, in order for boulders to continue to be moved in the stream or river, the water must be moving at a velocity of at least 200 cm/s.

   The transported sediment will weather (wear away and break down) the surfaces that they flow over. The stream’s channel will become deeper as the sediments carve deeper into the stream channel. (Recall, that this process where the stream channel is made deeper, is known as downcutting). As the sediments collide, pieces of the sediment are broken off and larger sediments create smaller sediments. The sediments continue to rub and grind against the surrounding rock. This process where rocks are worn away by the crushing and grinding of other sediments, is known as abrasion. As the rock is worn away, the resulting rock is rounded and smooth.

   

   The weakest rocks and sediments are eroded first. Waterfalls are created during this process. The water continues to flow over the the hard, resistant bedrock, and erodes the soft, non-resistant rock.