Welcome to Del Mar Dog Beach! Even if you don’t have a dog, this beach is sure to provide some fascinating entertainment. Low tide is the best time to visit this beach, and beware of large groups of dogs on the beach! They can unpredictably rush towards you and can even knock you over if they are big enough! Use caution, and please remember to respect other people’s pets, even if they come up to you first.
This spot along Del Mar Dog Beach contains the inlet for the San Dieguito Lagoon, which extends inland and is fed by the San Dieguito River. The tides of the ocean play an important role in continually shaping the entrance of the San Dieguito Lagoon. When a coastal lagoon is completely sealed off from the ocean, it becomes a coastal lake, which can be brackish if it was once filled by sea water. However, coastal lagoons with tidal entrances are estuarine in nature; transition zones between ocean and freshwater systems. As we can see here, there is a clear tidal inlet to the San Dieguito Lagoon.
This point of exit for the lagoon (or entry, for sea water!) is always changing shapes and sizes due to the ebb and flow of the tides. When the tide range is small, the entrance to this lagoon may be periodically closed off, as there would not be enough tidal force to continually push sand and other sediment out of the way. Similarly, if the outflow from the lagoon is not strong enough to push its own way out, the lagoon can become sealed-off completely. During high tide ranges, the waves will come in far enough to erode away sediment and build a connecting channel from the ocean to the lagoon. The tides can more easily move the loose, unconsolidated sediment rather than hard rock that has been in place for many years. Where the looser sediment lies plays an important roll in the route that the outflow channel takes, since water takes the path of least resistance. This can result in different shapes and angles being eroded throughout time.
To log this Earthcache, send me a note with the following:
1. Name of this cache at the start, and the number of people in your group.
2. Observe the current shape of the tidal inlet. Why do you think its wider towards the ocean, and skinnier as it goes inland?
3. Knowing that its easiest for the tides to erode away looser sediment than harder rock, what main feature (or lack thereof) explains why the inlet formed here rather than 100 yards north?
Sources:
https://gotbooks.miracosta.edu/fieldtrips/index.html
https://link.springer.com/referenceworkentry/10.1007%2F3-540-31060-6_55
https://en.wikipedia.org/wiki/Estuary
https://www.sciencedirect.com/topics/earth-and-planetary-sciences/tidal-inlet
http://www.coastalwiki.org/wiki/Morphology_of_estuaries