The Connecticut River is the longest river in New England, running about 410 miles from the Connecticut Lakes on the Canadian border, to the Long Island Sound near Old Saybrook. The river's watershed includes an area a little larger than 11,000 square miles in Vermont, New Hampshire, Massachusetts, and Connecticut. The river also forms the border between Vermont and New Hampshire.
The river sits in a valley that was partially created when North America split from Africa in the Triassic period. Several fault lines, called the Newark Supergroup were created as the continents began to split apart, resulting in a series of gashes in the local highlands. During the Ice Age, glaciers carved the bedrock even further, and created a large lake in the northern part of the river. As the glacier receded, the ground that held this large lake back let go, and caused a flood that created the river as it exists today.
Several proto-Indian peoples arrived in the area 11,000 years ago to hunt the animals that returned after the glaciers receded. These first people would mature into tribes with permanent settlements about 1500 years ago. These were the Algonquin tribes, and they called the river Quenticut, or "the long tidal river".
In 1614, Dutch explorer Adriaen Block became the first European to enter the valley. He called the river "de Versche" or Fresh Water, as finding the river afforded him an opportunity to replenish his supplies. He created the House of Good Hope on the site of present-day Hartford as a trading post, and the short-lived Kievits Hoek trading post at the mouth of the river. The British began settling the valley in 1630, bringing disease to the local population and greatly reducing their numbers. The river was very important to these early settlers, as it was the only convenient route for goods to come down the river, and for settlers to travel inland.
After several conflicts, the Dutch ceded the valley to the British, who promptly stepped up settlement of the area. Soon, these settlers made their way up the river into present-day New Hampshire, displacing local tribes there as well. While originally subsistence farmers, the settlers soon began to trade excess with the local tribes and Europe. Furs, tobacco, and maple syrup were the major exports. Soon large towns developed around the trading posts on the river, including the cities of Hartford and Middletown. When the South Hadley Canal was built to avoid the waterfall at South Hadley, Massachusetts, Springfield became an important city along the banks of the river.
The construction of the canal began a sting of 'improvements' to the river. The invention of the steam engine created a larger amount of ship traffic along the river, and thus necessitated some changes to the river in order to accommodate these ships. More canals were built, including one at Enfield to avoid the rapids in the river there. Water wheels were built along the riverside in order to harness the river for industry. The towns of Holyoke and Turner Falls, Massachusetts were planned communities created with a dam that provided power to the towns.
As the industry along the river grew, so did the pollution in the river, and the ecology began to deteriorate. In the late 1800's, dams were built in order to provide electricity to nearby towns. These dams located along the river now prevented the Atlantic Salmon population from returning to their spawning grounds to reproduce, eventually eliminating all salmon on the Connecticut River. Pesticides from neighboring farms also found their way into the river, killing off many plant species. The 1950's brought the construction of the Interstate Highway System, which cut off many towns from the riverfront, as well as adding to the pollution of the river.
Today, there are many programs committed to cleaning the river of pollution, and reconnecting it to the towns that grew along the riverside. Cities such as Hartford and Springfield have undertaken large urban renewal projects creating parks and other access points to the river. Hiking and bicycle trails have sprung up throughout the watershed. The Atlantic Salmon has been reintroduced to the river, and many of the dams have added fish ladders to their structures, so that these fish may reach their spawning area. The ten recognized endangered species in and around the river have also started to return with the reduction of pollutants in the river system.
The improvements to the quality of the river have also allowed several non-native species of plants and animals into the river, and have started to choke out the native species. Fortunately, the Connecticut River has yet to be invaded by the Zebra Mussel that threaten the local ecology of the Hudson River and Lake Champlain.
Many towns and villages along the river have signed the "Connecticut River Compact" which demonstrates their commitment to "the betterment of the river valley." These towns will work together to protect the river from future pollution, and to preserve the ecology of the valley.
Resources:
http://www.ctriver.org/
http://www.bio.umass.edu/
http://www.portlandct.org/ | 
