This is an Earthcache – as such, there is no physical cache. Instead after examining the sediments in the stream below the bridge at Bluemont Park, you will answer 5 questions and message me the answers. This earthcache is in an Arlington public park which is open daily from sunrise to sunset.
It is NoVAGO's 20th Anniversary, and how do we celebrate anniversaries? With gifts, of course. For this year's geocaching series, hiders will each choose a yearly anniversary gift to base their hide around — drawn from the Traditional, Modern, Flower, or Gem gift lists.
For this cache I bring you Clematis, a flower symbolizes ingenuity, mental beauty, wit, and ambition. Known as the "Queen of the Vines," its habit of climbing high toward the light connects it to perseverance, focus, and intellectual strength. A stream is perseverant, as it wins against a hard rock not by having more strength, but by flowing all the time. In honor of that, I bring you to this bridge to observe the stream below.
Welcome to Four Mile Run, a 9.4-mile-long urban stream in Northern Virginia that drains roughly two-thirds of Arlington County before emptying into the Potomac River. This watershed serves as a practical example of the dynamic geological processes that continuously shape the landscape.
Sediment and Sediment Transport
Sediment is natural material produced by the weathering and erosion of rock. Gravity, along with moving fluids like water, wind, and ice, drives a process known as sediment transport. In marine and freshwater environments, currents and tides carry sand, silt, and other particles downstream. Once these materials reach a low-energy basin like the sea bed, they settle through sedimentation. Over geological time, these layers of deposits compact and cement to form sedimentary rocks such as sandstone.
Weathering vs. Erosion
Though often used interchangeably, weathering and erosion are distinct geological processes. Weathering involves the mechanical breakdown or chemical dissolution of rock in place, with no movement of the material. Erosion is the subsequent process where surface forces like water flow, wind, snow, or moving glaciers physically remove and transport those weathered particles to another location. Physical or mechanical erosion shatters rock into fragments, while chemical erosion dissolves minerals into a water solution that flows away. Depending on the environment, eroded sediment may travel only a few millimeters or thousands of miles.
Streambed Dynamics and Flooding
A streambed is the channel bottom of a creek, river, or stream that contains the normal water flow, while the lateral boundaries are called the banks. The bed extends up to the normal water line, and the banks rise above it, though this boundary fluctuates based on local water volume. Streambeds vary drastically in scale, ranging from a few feet wide in small creeks to several thousand feet in major world rivers. Under certain conditions, a river may split from one primary channel into multiple paths, a phenomenon known as river bifurcation. When water levels exceed the capacity of the banks, the stream enters a flood stage, submerging the banks and overflowing onto the adjacent floodplain. Even if a stream dries up completely, the original bed is often well-preserved because soft sand and debris fill the space between the harder, carved channel walls. These dry beds can store subterranean water or reactivate during heavy rainfall, playing a major role in stream rejuvenation.
Channel Armor and Hydrology
Stream armor occurs when a dense layer of surface pebbles, rocks, or boulders naturally lines a streambed or beach to protect it from rapid erosion. Humans mimic this natural defense by placing large boulders or manufactured concrete structures along shorelines, a protective barrier known as riprap. Geological modeling indicates that natural channel armor typically develops in high-energy environments prone to flooding. The river transports this heavy bed material through entrainment, specifically via suspension (floating in the flow) and saltation (bouncing along the bed). When these entrained sediments settle, they form a structured layer that directly alters the hydraulics and hydrology of the river. This underlying sediment layer acts as a physical barrier to the current, changing water velocity and distribution, which can ultimately force the stream to meander or carve out entirely new channels.
Geological Size Classifications
Geologists categorize sedimentary particles by their precise diameter to analyze their behavior in a landscape:
- Boulders: Rocks greater than 10.1 inches in diameter, typically too large to be moved manually by a person.
- Cobbles: Also referred to as cobblestones, these range from 2.5 to 10.1 inches in diameter.
- Pebbles: Small stones measuring between 0.07 and 2.51 inches in diameter.
- Gravel: A loose accumulation of rock fragments that spans multiple size classes, from small granules to large boulders.
- Sand: A granular material composed of finely separated rock and mineral particles, defined as finer than gravel but coarser than silt.
LOGGING REQUIREMENTS:
To log this Earthcache: Read the geology lesson above, observe the stream from the bridge at the posted coords, answer all 5 questions below. Easy, peasy. Answers can be sent via e-mail or messenger contacts on my Geocaching profile, just do it within a reasonable time. Group answers are fine. I just ask that whomever sends those answers should pretty please include all the members of the group AND it would be super nice if each person logging could reference the geocacher who sent the answers in their log."
QUESTION 1. How much of the armor in the stream is natural? (CHOOSE ONE)
A) 10% or less
B) 11% to 45%
C) 46% to 73%
D) 74% to 100%
QUESTION 2. What types of sediment do you see in the stream?
QUESTION 3. Did you see sediment being transported at the time of your visit? If so what was the largest size?
QUESTION 4. Is any of the natural sediment actively changing water velocity and distribution? Explain.
QUESTION 5. How different do you think this stream bed will be in 10 years?
OPTIONAL PHOTO: Posting a photo in your log readily indicates that you (and anyone else logging the find) are at the location.
Awesnap has earned GSA's highest level:
REFERENCES:
1. "Sedimentation and Armor of Long Quarter Branch", Earthcache by PhMJ, website, geocaching.com