Music sounds different when it’s played and heard outdoors. Especially at night. Especially on the water. Especially if you’re hearing Louis Armstrong for the first time.

Before black jazz was widely recorded, before New Orleans musicians had traveled widely, before there even was such a thing as radio, many listeners encountered jazz for the first time on the water. That was also where many future jazzmen first heard it, including Jack Teagarden and Bix Beiderbecke, who played briefly on the boats himself.

In the winter of 1918-19, Teagarden was first learning about this thing called jazz. There were rumors about a miraculous new cornetist from New Orleans, and it was known he was playing on excursion boats. So Teagarden and a few guys he worked with gathered on the wharf, where they knew Armstrong’s boat would be pulling in.

As the Streckfuss steamer made its way up the Mississippi, the sound of that incredible trumpet grew ever louder and more distinct. It was like nothing they had ever heard. By the time they were able to actually see Armstrong playing with Fate Marable’s band, Teagarden later reported, they all had tears in their eyes. Armstrong was everything he was cracked up to be and a whole lot more.

This story was told by Teagarden 20 years later to his band vocalist, David Allyn, who told it me about 50 years after that. It is also mentioned in “Jazz on the River” (University of Chicago Press, 229 pages, $27.50), a highly entertaining new book by William Howland Kenney, a professor of history and American studies at Kent State.

Mr. Kenney examines the phenomenon of riverboat jazz from a variety of perspectives: cultural, musical, and technological. He lays his story out both chronologically and geographically, following jazz up the Mississippi from New Orleans to Memphis to St. Louis, then turns up the Ohio to Cincinnati and Pittsburgh, offering a mini-history of jazz in each of these cities.

Mr. Kenney has a very specific point to make, and he does it eloquently and convincingly. The experience of jazz bands on the Mississippi riverboats in the years before World War II was an integral part of the development of American vernacular music. Just as importantly, without jazz, the riverboat excursion business wouldn’t have lasted as long as it did.

Once the boat companies lost the freight business to the railroads, they began to convert cargo ships into pleasure cruisers. Floating up and down the Mississippi was dreamy, but after a short while it became a mite too restful and just plain dull. The best thing to do was dance to a jazz band.

The fascinating thing is that the Streckfuss family, who were the dominant force in the excursion cruise business, didn’t just employ standard pop dance bands. They actually made a point of bringing on black orchestras that played genuine jazz. The key figure behind the movement (in more than one sense of the word) was Marable (1890-1947), a black pianist from Kentucky who like his bosses made his home in St. Louis, Mo.

Between the first decade of the 20th century and the war years, Marable was the Fletcher Henderson or Benny Goodman of riverboat jazz. Like Henderson he was an amazing talent scout, and he recruited dozens of future stars, to whom he gave fundamental training in ensemble playing and sight-reading. Louis Armstrong was merely the most remarkable of these men.

As Mr. Kenney shows, the river became an integral part of the American musical experience, and many songs of the 1920s employ old man river as an immediately recognizable symbol. Riverboat jazz empowered Armstrong to begin his career as an international traveler, but the trumpeter-singer would later channel his early experience into songs like “Lazy River,” “Dusky Stevedore,” “Shanty Boat on the Mississippi,” and “Lazy ‘Sippi Steamer Going Home.”

There have been several much-heralded and eagerly anticipated jazz books this year, but this small wonder (it has fewer than 200 pages of actual text) is one of the best. The only thing that’s missing – surprisingly – is a map. Many of us, especially provincial New Yorkers like me, are not as familiar with the routes of this nation’s byways as we ought to be.