Chesapeake and Ohio Canal National Historical Park
Chesapeake and Ohio Canal National Historical Park  is a United States National Historical Park located in the District of Columbia, West Virginia and Maryland in the United States of America. It lies in the Potomac River Valley between Washington, D.C. and Cumberland.
Stretching 184.5 miles along the Potomac River between Washington, D.C. and Cumberland, Maryland, the Chesapeake and Ohio Canal National Historical Park preserves remains of America's colorful canal era. For nearly a century, the C&O Canal was the lifeline for communities and businesses along the Potomac, as coal, lumber, grain, and other agricultural products gently floated down the canal to market. This magnificent water highway linked the rapidly growing west to the east and played an important role in the growth and development of our country.
The construction of Chesapeake and Ohio Canal started on July 4, 1828 and lasted for 22 years. The canal was completed on October 10, 1850 at a cost of about $14 million. The canal was 184.5 miles long, 6 feet deep and 60-80 feet wide. There were 74 lift locks built on the canal, 7 dams and 11 aqueducts. One of the most impressive features of the canal, the Paw Paw Tunnel is 3,118 feet long; 27 feet wide and 24 feet high. The canal was in operation from 1850 to 1924. In 1938 the government acquired the approximately 12,000 acres of land adjacent to the canal and Chesapeake and Ohio Canal National Historical Park was established in 1971.
The C & O Canal runs right along the Potomac River and the two couldn't be more contrasting. The canal is flanked by wide sandy paths for biking, walking, and jogging, and its water is still and green. On a nice day, the paths are full of people. Walk through the forest to the south just a few yards, and the mighty, rough Potomac River will roar in your ears. The Potomac is wide, fast, filled with boulders in and on the sides of the river, and surrounded by the dense woods that are the natural environment of the Mid-Atlantic. Views of the Virginia side's cliffs (closer to Washington, D.C.) can be especially pretty, rising above the rocky shore. Given that the canal runs alongside the river, the trails are mostly level.
Flora and fauna
You wouldn't guess it, but the C&O Canal National Park is one of the most biologically diverse in the national park system. The diverse landscapes of wetlands, streams, the river, forests, open fields, and rocky cliffs make for habitats for all sorts of plants and wildlife. Look in the mud beneath your feet and you'll find frogs, toads, salamanders, fish, freshwater mussels, beaver, and muskrats. Around in the woods, there are deer, song birds, red and gray fox, raccoons, gray and fox squirrels, and even the rare black bear and bobcat! Look to the skies for the most famous bird in this country, the bald eagle, which nests along the river alongside rocky cliffs, and is actually quite common along the river.
Be careful when wandering off the path, and avoid picking at the vegetation, because the C&O Canal is home to an extraordinary variety of rare, threatened, and endangered plant species. The region has so much rare plant life because of its idiosyncratic geography. The Potomac cuts through Maryland, known as "America in Miniature" for its radical shifts in geography from east to west. The Potomac cuts through the rolling woodland plateaus of central Maryland, then the Blue Ridge mountains, and further on the valleys of Appalachia, providing diversity of habitats for the plant life. This factor combines with the frequent flooding of the river, and the overlap in Southern and Northern species to make for very diverse and unique environments for plants to grow.
Given the region it passes through, the Canal and Potomac have a nice temperate climate. The only season that sees extremes is summer, when the temperature can rise as high as 100 degrees Fahrenheit, but even then the presence of the big river has a cooling effect. Spring is the loveliest time of the year here, as the wildflowers and dogwood trees show their colors every which way, among budding trees. Fall is a close second, due to the fact that the park is almost completely covered with colorful deciduous trees. Winter is a surprisingly good time to visit, because temperatures rarely drop below freezing, and because you'll have the park to yourself! And so do the animals, who shyly wander out into the open during the winter (since there are few people around to scare them off), and this is the best time of year to spot the rarer and harder to find species, like raccoons, muskrats, and the extremely rare bobcats. Summer is fun in its own festive way, as families set up picnics, bikers speed by, and all sorts of people converge on the park to check out its historic attractions and natural attractions like Great Falls.
This is a park that is extraordinarily easy to get to. Since it is one long trail, simply drive up to the Potomac River anywhere, and look for a parking lot (there are plenty).
At the Georgetown terminus, the canal is easy to find. Simply walk south from anywhere in Georgetown and you'll hit it. Then turn right and head west into the park!
Once in the park, no motorized vehicles are allowed, so you'll be on your own to get around the park. The modes of transport of choice are walking, jogging, and biking. Although it is also possible to kayak or canoe the Potomac through the park, make sure you know what you're doing—the rapids are very dangerous, especially around Great Falls, and people drown every year. Kayaking/canoing along the canal, instead, can be a much safer option.
Since the park is centered around one extremely long path, there is little opportunity for the park service to collect fees. But to entry to the heavily touristed Great Falls will cost $5 per private vehicle, $3/person on foot or bike.
The main sight is Great Falls an enormous collection of small waterfalls and rapids along a fast moving, but very wide stretch of the Potomac not far from Washington, D.C. Trails lead to ideal viewing points from both the Maryland and Virginian sides of the river, although the view from the island accessible from the Maryland side is more immersive.
Other sites of interest include old boats and canal locks along the canal, which you can find on display at the Great Falls area by the visitor center. The abandoned Gold Mine on the Maryland side near Georgetown is another interesting sight.
Aside from the Great Falls trail, there is, well, one other trail, which runs along for 185 miles. And it does not stop there, as it connects up with the Great Allegheny Passage, which leads all the way to Pittsburgh. So you won't run out of space to bike or jog, although you'll likely run out of steam before completing the trail!
Each visitor center has a bookstore, where you can find books about the park, its physical and living features, as well as books on the history of the region. You can also shop for the same books online. 
Eat and drink
There are no facilities serving food throughout the entire park, so pack in and pack out. Possession of alcoholic beverages in the park is against the law, and can result in a hefty fine. Permits for alcoholic beverages can be secured, but usually only for events being held in the park, like weddings.
There are five drive-in campsites, which are pretty basic: just improved tent pitching sites, grills, chemical toilets, and potable water. There are no RV hook-ups, and RVs are only permitted at three of the five. $10/night.
There are 30 free hiker/biker campsites with stays limited to one night per site. All sites have a chemical toilet, water, a picnic table, and grill. Drinking water is not always available, so be prepared to purify your own.
There is really just one danger and one danger alone in the park—the river. The Potomac is a huge and fast river, with plenty of rapids. Drownings occur every year, usually as people foolishly try to go wading in what looks like calm water, and are promptly sucked under by the strong current. Kayakers, rafters, and canoers would do well to consider this fact as well.
There are a couple major sites of interest just off the trail you should consider visiting. Among the most important are Harpers Ferry, just across the Potomac in West Virginia, and Antietam National Battlefield. And the terminae Cumberland and Georgetown both make for nice places to relax and get some good food.