Our journey through the freshwater system of wetlands, lakes and rivers has brought us to the place in our watersheds where the rivers meet the seas and oceans.
The presence of fresh water and marine salt water in the same place makes a unique and important habitat. The mix of nutrients and sediments here allows some of the most productive ecosystems in the world, not only for plants and animals, but also for humans. The geographical terms for these mixing grounds of fresh water and salt water, found at the mouths of the rivers, are estuary and delta, both important and geographically very different.
A delta is formed when sand and silt are deposited at the mouth of a river, often in a triangular or fan-shaped land mass, such as the deltas on the Nile and Mississippi rivers. Estuaries are inlets or arms of the sea where the salty tide meets the freshwater current and are highly complex aquatic ecosystems, such as the Penobscot Estuary in Maine. Deltas and estuaries are characterized by a high number of plants and animals.
In addition to deltas and estuaries, two natural habitats found along the coastline are salt marshes and mangrove forests. Mangrove forests grow at the edge of land and sea, mostly within 30 degrees of the equator and are tropical, although there are some hardy types that have adapted to temperate climates and one that is as far south as New Zealand. Mangrove populations share an ultrafiltration system to keep much of the salt out of the plants and a complex root system that allows it to survive in the intertidal zone.
The two ecosystems of salt marshes and mangrove forests support numerous species of plants and animals, including mollusks, crustaceans, otters, dolphins, manatees, crocodiles and sea turtles. They support migratory bird populations and are important transition zones for salmon, shad and eels.
These coastal wetlands serve the same functions as our freshwater wetlands in that they are nurseries for a variety of species and act as natural filters through their plant life, which absorb and control the nutrient concentrations, preventing them from reaching toxic levels.
Many of the world’s major cities have been built at the mouths of rivers. Today, more than 60 percent of the world’s population is located within 62 miles of a coast.The entire country of Bangladesh rests on one of the largest deltas in the world, and some of the world’s largest cities, such as Tokyo, Calcutta, Yangon and Alexandria, are along deltas. There are approximately 40 deltas worldwide, with the largest being the Amazon River delta. These deltas are important to people for agriculture and fishing and as ports and places to live.
Estuaries are the aquatic environments at the mouths of rivers. There are different physical types of estuaries. The Penobscot River Estuary, which goes from Bangor to Searsport-Bucksport where it meets Penobscot Bay, is an example of a drowned river valley. The river began in prehistoric times with the glacial meltwater, and carved a valley through granite and metamorphic rock, later to be inundated by the rising post-glacial sea. There are two other types of estuaries. One is formed when an offshore bar develops along a relatively flat shoreline, such as the barrier island system along the coast of Texas; and the second type is a deep-basin estuary, such as the fjords of Canada, Norway and New Zealand.
The Kennebec Estuary in Maine is a good example of the diverse ecological systems that are found in estuaries. It contains more than 20 percent of Maine’s tidal marshes, a significant percentage of Maine’s sandy beaches and associated dune habitats, as well as rare pitch pine woodland communities. Six rivers come together in the upper Kennebec Estuary. They are the Kennebec, Androscoggin, Cathance, Abagadasset, Muddy and Eastern, all of which converge in a shared delta to form Merrymeeting Bay. Merrymeeting Bay is one of the few places in the world where multiple major rivers come together to form an enclosed tidal delta. One plant, Long’s bittercress, has been found in only one location in the Kennebec Estuary. The abundant freshwater flow from the six rivers is one of the largest freshwater tidal estuaries in the eastern United States, and therefore one of the best habitats anywhere in the world for the rare plants found there.
Maine’s 3,500 miles of coastline also has numerous salt marshes. A salt marsh is a land area flooded by the sea. Salt marshes filter pollution from the water and provide habitat, food, water shelter and space, for many species of birds, mammals and fish, and shellfish such as lobsters and crabs. The Scarborough Marsh covers more than 3,000 acres and is the largest contiguous salt marsh system in Maine.
These places where the rivers meet the seas are critical habitats for many species, including us. Humans depend on the plants and animals of these areas for their livelihood, for transportation and as a source of biodiversity. Unfortunately, overuse of land for recreation and development is destroying these fragile and unique habitats. Many organizations and individuals are working to help educate others on the importance of these areas.
Joanne DeFilipp Alex is the northern Maine geography awareness coordinator for the National Geographic Society. This article was made possible with support from the Maine Geographic Alliance.
Project WILD Aquatic K-12Curriculum &Activity Guide, Council for environmental Education
Use the news:
Find articles in today’s paper about the coastal environments discussed in this article. What were the topics of the articles? Look for the terms estuary, delta, salt marsh and mangrove forest. Locate the places you find on a map. Are they local, regional or global issues discussed?
1. Locate the places mentioned in the article on the map.
2. Find out more about the different types of estuaries mentioned in the article. Where are the different types located on Earth? Is there a correlation between location and type?
3. Locate the deltas on a map and the cities that are located on or near deltas. Research the cities. Where are they located geographically?
4. Research salt marshes and mangrove forest habitats. What are the similarities and differences?
5. Pick one species that lives in these environments where the rivers meet the seas and find out more about it. Could it survive in a different type of habitat as well?
6. Choose a delta, such as the Colorado River Delta, and research what challenges it faces today.
Monty has packed his bags and will travel to the largest of the 130 estuaries in the United States to take in the sites as well as enjoy the flora and fauna of the region. In this estuary, half of the water comes from the Atlantic Ocean, the other half from a 64,000-mile watershed, including the Susquehanna, Potomac and James rivers. Where is he? (Answer: Chesapeake Bay, USA) http://www.chesapeakebay.net/estuarysystem.aspx?menuitem=14605)