Note: This is an Earthcache. There is no physical cache to find. Logging this Earthcache requires that you undertake an educational task relating to the specific Earth Science at the site.
Prior to logging this cache, click on Message this owner, or send an email with answers to the following questions:
- Geologically, what type of estuary is this?
- Based on circulation, what type of estuary is this?
- The Tijuana Estuary covers how many acres of the Tijuana River drainage basin?
- How many of these acres are salt marsh?
- Do you think this immediate area is part of the salt marsh?
- Required to log this cache:Please provide a photo of yourself or a personal item in the picture to prove you visited the site. Upload the photo with your log.
The answers can be found within this description and at the posted coordinates.
Tijuana River National Estuarine Research Reserve
The Tijuana River National Estuarine Research Reserve is made up of freshwater from the Tijuana River that connects with the saltwater of the Pacific Ocean, creating a large wetland habitat dominated by low lying vegetation known as a salt marsh. This connection of waters happens at the river mouth, and because of it, the water level in the estuary rises and falls with the ocean tides.
The Tijuana Estuary is one of the few salt marshes remaining in Southern California, where over 90% of wetland habitat has been lost to development. The site is an essential breeding, feeding and nesting ground and key stopover point on the Pacific Flyway for over 370 species of migratory and native birds.
The Reserve offers four miles of walking trails, taking visitors into prime bird watching areas and down to the river mouth where the Tijuana River meets the Pacific Ocean. Visitors may explore the park on their own or join one of the free guided nature and bird walks on weekends.
You can find several definitions for an estuary:
- An estuary is a semi-enclosed body of water which has a free connection to the open sea and within which seawater is measurably diluted by fresh water derived from land drainage.
- An estuary is a coastal wetland where freshwater that flows from rivers and streams mixes with the saltwater from the ocean.
- An estuary is the thin zone along a coastline (such as bays, lagoons, sounds or sloughs) where freshwater systems and rivers meet, and mix with a salty ocean, becoming brackish
- An estuary is the endpoint of a watershed.
- Or simply, an estuary is where a river meets the sea.
Estuaries are classified based on two characteristics: their geology and how saltwater and fresh water mix in them (circulation).
The five major types of estuaries – classified by their geology:
- Coastal Plain – Millions of years ago, as ancient glaciers melted, some coastal streams and rivers became covered with water as sea levels rose. The Chesapeake Bay in Maryland and Narragansett Bay in Rhode Island are examples of coastal plain estuaries that were once river valleys.
- Bar-built – Sandbars or barrier islands built up by ocean currents and waves in coastal areas created a protected area fed by small streams or rivers. The barrier islands off the Atlantic coastline of North Carolina and Massachusetts enclose bar-built estuaries
- Delta system – Deltas are formed at the mouths of large rivers from sediment and silt depositing instead of being washed away by currents and waves. When the river flow is restricted by the delta, an estuary may form. The estuaries at the mouth of the Nile River in Egypt and the Mississippi River in Louisiana are examples of delta systems.
- Tectonic – Tectonic estuaries were created when a major crack or a large land sink in the Earth, often caused by earthquakes, produced a basin below sea level that fills with water. These types of estuaries usually occur along fault lines. San Francisco Bay in California is an example of an estuary created by tectonics.
- Fjords – Advancing glaciers ground out long, narrow valleys with steep sides. Then when glaciers melted, seawater flooded in. Glacier Bay in Alaska is an example of a fjord.
The six major types of estuaries – classified by their circulation:
- Salt wedge - River output greatly exceeds marine input; there is little mixing, and thus a sharp contrast between fresh surface water and saline bottom water.
- Highly stratified - River output and marine input are more even, with river flow still dominant; turbulence induces more mixing of salt water upward than the reverse.
- Slightly stratified - River output is less than the marine input. Here, turbulence causes mixing of the whole water column, such that salinity varies more longitudinally rather than vertically.
- Vertically mixed - River output is much less than marine input, such that the freshwater contribution is negligible; longitudinal salinity variation only.
- Inverse estuary - Located in regions with high evaporation, there is no freshwater input and in fact salinity increases inland; overall flow is inward at the surface, downwells at the inland terminus, and flows outward subsurface.
- Intermittent estuary - Estuary type varies dramatically depending on freshwater input, and is capable of changing from a wholly marine embayment to any of the other estuary types.
Geologic origin of the Tijuana Estuary
As continental drift shifted North America toward the west, a steep coastline and narrow continental shelf developed. Marine terraces were gradually carved along the shores. Then, in the late Cenozoic, tectonic uplift raised alluvial terraces to several hundred feet above modern sea levels. What is now the Tijuana River, presumably cut through these terraces, although the narrow floodplain suggests that flows were not consistently large.
In the Holocene, a rising sea began to reclaim the exposed margins of the coastal shelf. Rivers were drowned and lagoons formed as longshore drift created sandy barriers along the coast, making this a bar-built estuary, shaped by underlying tectonic factors. With flooding, most of the coastal embayments filled with sediment. Without continuous river flow and scouring, their mouths closed between flood seasons.
Recent geologic factors that have shaped the estuary are the competing forces of rising sea level, which promotes inland migration of the estuary, and tectonic uplift, which reverses that trend. The location of the shore and the configuration of the mouth are additional variables that influence the size and condition of the estuary.
Circulation in the Tijuana Estuary
The Tijuana Estuary is a small intertidal coastal estuary on the international border between California and Mexico. The estuary is primarily a shallow water habitat, though it is often termed an intermittent estuary, as it is subjected to extreme changes in streamflow at different times of the year. Extended periods of drought leave parts of the estuary dry during some periods, while flooding inundates the same areas during others. For this reason, Tijuana Estuary is considered to be a very unique part of the National Estuarine Research Reserve System.
Streamflow in the San Diego region is the most variable in the United States and differences between wet and dry years at the reserve are greater than in any other part of the country. On average, the Tijuana River has its peak flow in March. The Tijuana River can experience many months with no flow. Year-to-year flows are highly variable, as are monthly flows. This estuary is Southern California's only coastal lagoon that is not disturbed by roads and rail lines.
Sources:
The Encyclopedia of Earth - Tijuana River National Estuarine Research Reserve
California State Parks – Tijuana Estuary NP
