Getting to know the River
You are currently standing at the mouth of the Pasagshak River, which runs only three miles and connects Lake Rose Tead with the nearby Pasagshak Bay.
Lake Rose Tead
Prior to the Great Alaskan Earthquake of 1964 the nearby Lake Rose Tead (visible to the North) was a completely fresh water lake which was sourced solely by mountain runoff and supplied water to the Pasagshak River, however the seismic magnitude experienced on the 27th of March in 1964was so strong that it lowered the lake by nearly 1.7 meters (Murray, 1986). Now at sea level, Lake Rose Tead receives tidal saltwater intrusion via the river which has created a rich marine estuary system which connects the freshwater coming from the mountains with the salty ocean water from the nearby bay. This brackish environment is supported by the Pasagshak River; a tidal river.
The Tidal Rivers
A tidal river is the part of a river-estuary system where there are strong interactions between tides and river flow. River processes increasingly dominate over tidal processes farther upstream (Moldwin, 2017). This is a little studied phenomenon amongst scientists, as it involves the collaboration of Oceanographers, Hydrologists, and Geologists. Due to the lack of appropriate scientific studies the actual portion of any river that can be refered to as a "tidal river" is ambiguous and ill definied (Hoitinik, 2016), however the basic premise agrees that it must be an area that is predominantly governed by freshwater flow and be sucepitble to influences of tidal forces. Standing where you are you are in that special area of the river.
The Mouth of the River
Where the Pasagshak River meets the Bay of the same name you can see the tidal forces interacting with the normal flow of the river. As freshwater attempts to drain seaward the tides will either work in the same direction as the forces in the river or will combat them. If the tide is coming in and opposing the flow of the river it will increase turbidity at the mouth of the river. This will be easily seen by larger waves and rougher waters. This effect is felt up the river and can actually slow the rate of freshwater drainage from Lake Rose Tead. In comparison, if the tide is going out, the river flow can increase and the mouth of the river will appear relatively calm (in the absence of an outside factor like winds).
Black Sands
Typically when you think of Black Sand beaches you imagine the picturesque shores of a tropical island. While not tropical in nature the sands along the Pasagshak River are comprised of a similar material. As noted by Meneghini the black sands are created during tectonic processes which impose incredible forces on the existing cataclastite. Cataclastite rocks are formed during the faulting process and are typically fine grained and formed from volcanic activity. So just like the sunny beaches of Hawaii, the black sands of Pasagshak are formed from violent volcanic and seismic activity. The sands at your feet are eternal reminders of the volcanic past of Kodiak Island.
Logging this Earthcache
As with all Earthcaches, in order to claim your find you must send me the answers to the below questions which help demonstrate your comprehension of the earth science lesson. Answers may be sent to me via the message center. You do not have to wait for a reply from me to log your cache; I will contact you if I notice any discrepancies with your answers.
1) How would you describe the sands at this location? Are they fine, coarse or gravel sized? What does this tell you about the erosion processes here?
2) Estimate the size of the river delta forming across the river from your current location (both overall width and length). Is the fact that this river is tidal increase or decrease the rate that the delta is forming? Why do you suppose that is?
3) What correlations can you draw between the size of the grains of sand and the conclusions you drew in question "2?"
4) (Optional) Post a photo of yourself at the river. Tell me what brings you out to this part of the island!
Sources
1) Tidal River Dynamics by Mark Moldwin (2017)
2) Research Project F-10-1; S-41 by John Murray (1986)
3) Tidal River Dynamics; Implications for Deltas by A. Hoitink (2016)
4) Record of mega-earthquakes in subduction thrusts: The black fault rocks of Pasagshak Point (Kodiak Island, Alaska) by F. Meneghini et al (2010)
CONGRATS TO DENALI DUO ON THE FTF