Did you know the stream channel down to the river didn't use to look like this? In recent memory (the past 20 years), the stream was a small bed funneling down to the river without a chasm around it. The chasm is a relatively recent introduction to the landscape. During development of the new subdivisions in Lower College Heights, water from the areas being built was increasingly channeled into waterways that increased the flow of water into this stream. The increased volume of water greatly increased the erosive power of the stream, which began a rapid cutting action into the ground. This erosion resulted in first a steep-walled chasm from the top of the gorge down to the riverbed - a dizzying and dangerous drop. The sides of the creek began weakening, resulting in the stabilization efforts visible today to protect the surrounding area.
Types of Erosion
There are two main types of erosion - mechanical and chemical. Mechanical erosion is where there is a physical contact between the erosive forces and the object being eroded. Chemical erosion occurs when a chemical decays the material being eroded. Moss and lichen are responsible for the chemical erosion of large rocks, to break them down so mechanical erosion by wind and water forces can reduce even the largest of rocks to pebbles or sand (eventually!).
This cache will only be examining mechanical water erosion.
What is Erosion?
Sten [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], from Wikimedia Commons
Erosion in geological terms is "the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transport it away to another location" (Wikipedia, Erosion, May 6, 2018). Erosion involving physical contact between wind or water and the rock or soils is a mechanical erosion (ibid) as opposed to a chemical erosion. In this instance, the soil of the area has been removed from the land by the moving water of the stream and transported into the river. When we compare the size of the stream to the size of the chasm, the water doesn't appear significant enough to threaten the landscape.
Results of Erosion
Erosion can cause some pretty neat results. Arches and hoodoos are both the results of erosion.
Arches are caused when the erosive action of water carves out the inner core of a rock material while eroding the hard outer materials less. The result looks like Arches Provincial Park, in Newfoundland. (As a great follow up on erosion, you can learn more about rock arches (or rock bridges) at https://en.wikipedia.org/wiki/Natural_arch.
By David W Enstrom [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons
Available closer to home are hoodoos. Pinnacles Provincial Park in Quesnel, BC, has pillar-style rock formations where erosive forces have carved out the landscape around the hoodoos. There's an earthcache there - The Pinnacles GC348R7.
Not all erosive results, however, are beneficial. Erosion can result in the loss of the most nutrient-rich layers of soil, making the soil less productive for plant growth. Eroded sediment carried by the water is often deposited in the waterways, causing sedimentation problems and eutrophication (Wikipedia, Erosion, May 6, 2018). Urban sprawl is recognized as one of the major factors in stimulating erosion, in part due to the concentration of higher water volumes into fewer waterways and allowing water to move faster in the waterways (ibid).
In this instance, the erosion simply carved away the landscape. Due to the soil composition of the area, stabilization efforts had to be taken to prevent further erosion into the river.
Ways to Prevent Erosion
Erosion can be prevented by limiting the movement of the soil particles, slowing down the flow of water through the area, or providing a stabilizing surface that erodes much more slowly.
Methods used to prevent soil erosion are:
1) Planting vegetation - the vegetation will take root, and bind the soils in place so it is no longer free to move with the water.
2) Installing geotextiles - geotextiles help stabilize the soil when they are installed, preventing the soil from moving with the water.
3) Applying mulch or fertilizer - this method attempts to absorb excess water into the mulch or fertilizer to prevent the water from running.
4) Install retaining walls - artificial barriers can be installed to prevent soil from moving.
Retaining walls can be temporary or permanent. In construction zones, silt fences are installed as retaining walls to slow the flow of water and stop the soil from moving, while physical barriers such as hay bales can be placed in drainage ditches to mitigate the speed of the water flow. Below is an image of a silt fence installed to prevent soil runoff.
By U.S. Environmental Protection Agency (EPA) (U.S. Environmental Protection Agency]) [Public domain or Public domain], via Wikimedia Commons
Local construction often uses Green Tac, or Hydroseed, as a means of attaching seed to stripped land. The Hydroseed includes a fertilizer to encourage the plant growth, with the goal of having the plant roots stabilize the soils from runoff. Hydroseed is made from "biodegradable glue, grass seed, newspaper pulp, lime and fertilizer. It is mixed with water at the time of application, and given an environmentally safe greenish color to show where the application has already been applied." (Amy, 2009, May 06, 2018). The fertilizing agents will also help limit the runoff by absorbing water in the soils.
By Projar [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons
The use of large less-erosive materials to line the bed or shores of waterways can prevent erosion. For many bridges, roads, etc, large rocks are used to fill erosive areas. This material is commonly known as rip-rap. Very common along coastal areas, rip-rap has also been used to shore up local bridges such as the Salmon Valley bridge (an area prone to the river rerouting itself). The rip-rap is dropped along the contact edge between the water and the eroding land so the rip-rap will take the force of the water and prevent the erosion of more mobile sediments.
Riprap {{PD-USGov-Military-Army-USACE}} source: http://www.orn.usace.army.mil/cof/Copy%20of%20Copy%20of%20MVC-021S.JPG
To log this earthcache, please email or message me the answers to the following questions. LOGS WITHOUT ANSWERS WILL BE DELETED, AS PER EARTHCACHE CRITERIA.
1) Estimate the width of the chasm (at the top) at its widest point.
2) What mitigation strategies can you see along the stream to prevent further erosion? (Minimum of two)
3) What evidence can you see of ongoing or current erosional activities?
4) Why do you think the riprap extends for as much of the creekbed as it does?
5) Photos are not required, but certainly spice up a log and make things more interesting. :-)
References:
GEI Works. (2011). Erosion Pollution. Accessed May 06, 2018 from https://www.erosionpollution.com/ways-to-prevent-soil-erosion.html.
Amy. (Oct. 2009). Green Gardenista: What is the Green Stuff Construction Workers Spray on Newly Developed Areas? Accessed May 6, 2018 from http://www.greengardenista.com/blog/what-is-the-green-grass-seed-spray-construction-workers-use-on-newly-developed-areas/.
Wikipedia. Erosion. (Last updated Apr. 2, 2018). Accessed May 06, 2018 from https://en.wikipedia.org/wiki/Erosion.