Today’s Wonder of the Day was inspired by Jenni from Imperial Beach, CA. Jenni Wonders, “What is the longest river on Earth?” Thanks for WONDERing with us, Jenni!

Can you imagine what it would have been like to grow up in the days when Abraham Lincoln was young? The Southern Indiana hills he called home were still wild and considered the frontier to the families who settled them.

As a youth, Lincoln operated a ferry across the Ohio River for a short time. He also took a long flatboat journey down the Ohio and then all the way down the Mississippi River to New Orleans. These river adventures helped to shape him into the leader he would eventually become.

The great thing about rivers is that they offer plenty of opportunities for adventure. While many people choose to use automobiles and airplanes to get to their destinations, it's still possible to navigate your way down rivers by boat.

If you look at a map, you'll see there are thousands of rivers that snake their way across the globe. These rivers offer not just tens or hundreds of miles of fun, but thousands of miles of natural excitement. If you're up for a challenge, just try setting sail on the world's longest river.

But which river would that be? Although it might seem like a simple question, the answer isn't exactly straightforward. Determining how to measure rivers and then deciding upon exactly where a particular river starts and ends are not exactly easy tasks that all scientists can agree upon.

For example, most of the world's major rivers are actually part of larger river systems, so it can sometimes be difficult to decide where a specific river begins. Most scientists today choose to measure the length of the longest continuous river channel in a particular system.

A river's size can also be measured in terms of its magnitude and volume, rather than simply its length. Crowning the "largest" river, therefore, might not be the same thing as determining the "longest" river.

Despite these difficulties, most scientists agree that the Nile River is the longest river in the world. Flowing north through eastern Africa and emptying into the Mediterranean Sea in Egypt, the Nile River travels an amazing 4,258 miles. Of course, given the difficulties in measuring rivers, some estimates put the Nile's length at about 4,135 miles.

Regardless of which measurement you use, the Nile River is still longer than its nearest competitor, the Amazon River, which flows through the South American countries of Peru, Ecuador, Colombia, and Brazil. The United States National Park Service estimates the length of the Amazon to be 3,980 miles long.

Brazilian scientists who have taken new measurements of the Amazon recently believe it should be considered the world's longest river at 4,345 miles. However, their work hasn't been validated by the rest of the scientific community yet.

Even if it's not the longest river in the world, the Amazon can still boast. Not only is it the widest river in the world (some parts of the Amazon are 120 miles wide during the rainy season), it's also the world's largest river by volume. In fact, the Amazon contains approximately 20% of the world's fresh water supply!

Wonder What's Next?

Tomorrow’s Wonder of the Day will really put your brain to the test!