This earthcache will take you to two spots along Coyote Creek in downtown San Jose. Both locations are free to visit, each a short walk from separate parking spots, and accessible from one hour before sunrise until one hour after sunset. The parking spots for either stage are about a 6 minute drive from each other – there is no direct route between the two stages. You will be learning about the effect of human development on river floodplains and how it relates to the evolution of oxbow lakes. Please visit both sites and answer the logging requirements at the end of the description to claim credit for this earthcache.
Coyote Creek is actually a river which runs over 60 miles through the heart of Santa Clara Valley, starting on Mount Sizer in the Diablo Range and flowing north until it empties into the San Francisco Bay. The river is impounded by two dams, first Coyote Reservoir and then Anderson Lake. Over the years, the water level of Anderson Lake has been purposefully lowered due to safety concerns, with the most recent change being in 2020, when the reservoir was drained to 3% of its capacity. The primary concern stems from the fact that the Coyote Reservoir Dam is built directly on top of the Calaveras Fault. A seismically-triggered landslide could cause the failure of the upstream dam, leading to a sudden release of water which would then increase the risk of the Anderson Dam failing.
After Anderson Lake, Coyote Creek enters the broad, flat Santa Clara Valley. In this type of lowland area with lots of open space, rivers tend to form meanders. Meanders are the horseshoe-shaped bends that form in response to a pressure differential between the two sides of a river channel. Sediment deposits on the inside of the bend, and erosion removes sediment from the outside of the bend (see diagram below). The result of these combined effects is that meanders grow through time, with a typical timescale of tens to hundreds of years. The culmination of an evolving river meander is what’s known as an oxbow lake. A meander will become so pronounced that a narrow neck of land will be all that remains between two neighboring outer (i.e. eroding) banks. During a rainy season, particularly strong currents or even flooding finally cuts the neck, resulting in both a more direct river channel and the abandoned meander loop, referred to as a cutoff. Afterwards, the deposition of sediments will then seal the cutoff, creating an oxbow lake.
The posted coordinates will bring you to the outer (i.e. eroding) bank of a particularly sharp meander along Coyote Creek. A cursory glance of satellite imagery will show a number of meanders along Coyote Creek, but what sets this one apart is the existence of a shallow channel east of the meander at the Stage 2 coordinates (along the Coyote Creek Trail). This shallow channel is the river attempting to straighten its course by circumventing the narrow outer meander. Essentially, we have the opportunity to view an intermediate stage of oxbow lake evolution at this site. Equally important, we also have the opportunity to view how human development affects oxbow lake evolution.
As a result of the water demand from a large metropolitan area such as Santa Clara Valley, the water levels and flow rates of local rivers are significantly lower than prior to human settlement. Coyote Creek is especially affected, as it is dammed twice upstream, meaning that very little source water makes it to the San Francisco Bay. Lowland river evolution is directly proportional to the river flow rate, as both sedimentation and erosion both rely on deep, fast waters. Thus, river evolution, including the formation of oxbow lakes, has stalled in the area over the past century. The Coyote Creek floodplain has changed little in recent history, as a result of both the decreased water flow rate and the nearby urban development, much of which is directly adjacent to the river.
Logging requirements:
Send me a message (preferred) or an email with answers to these questions:
1. The name of this earthcache "Stalled Oxbow Lake Evolution Along Coyote Creek," its GC code GCB8R9Q, and the names of all geocachers in your group.
2. At Stage 1 (the posted coordinates in Rocksprings Park), look through the fence to make estimates of the following observations of Coyote Creek. (hint: the "channel" is entirely below your current ground level and on the other side of the fence):
a) Channel depth
b) Channel width
c) River water depth (if any)
3. At Stage 2 (along the Coyote Creek Trail), make estimates of the following observations of the shallow channel you are standing in (hint: the "channel" is precisely at the Stage 2 coordinates and entirely below the level of the trail):
a) Channel depth
b) Channel width
c) Water depth (if any)
d) Approximate elevation difference between 50 feet directly west and 50 feet directly east of you. In which direction is the land higher?
4. Why do you think the shallow channel is located at the coordinates for Stage 2 and not farther east, where the river’s course would be even more direct (think about your answer to 3d)?
5. What effect do you think human development in Santa Clara Valley has on the evolution of an oxbow lake at this site? Do you think an oxbow lake will ever form here, taking into account the current human impacts?
6. What effect do you think an upstream dam failure might have on the meander here? This doesn’t necessarily have one correct answer, so I’m asking how you think the sudden influx of water would interact with both the meander at Stage 1 and the shallow channel at Stage 2 (hint: think about how oxbow lakes form).
7. Post a picture of yourself or your signature item at the posted coordinates in your log. Do not include answers to the logging requirements in your picture.
Please Note:
1. You must answer all the questions in your message/email to me.
2. Your logging of this cache and messaging/emailing me should happen at around the same time.
Failure to comply with the above will result in your log being deleted!
The above information was compiled from the following sources:
- http://tecto.caltech.edu/member/outreach_resources/host_caltech/scheingross/sci_sat_program_final.pdf
- https://www.bbc.co.uk/bitesize/guides/z6jx382/revision/4
- https://www.geographyrealm.com/how-oxbow-lakes-form/
- https://en.wikipedia.org/wiki/Coyote_Creek_(Santa_Clara_County)
- https://www.sciencelearn.org.nz/resources/440-human-impact-on-rivers