Urban canoeing had become a popular activity in many US cities in the late 19th century. Boston's Charles River became a well-known destination for leisurely trips on the water, since the boathouses in Newton were accessible by train and trolley from all over the city.
One of the most popular of these sites was Norumbega Park which had been built in 1897, a year after the trolley running down Commonwealth Avenue in Newton had been completed. (The Marriott Hotel now stands at this location.) The park offered canoeing, a zoo, a merry-go-round, picnic spots, a restaurant, and a vaudeville theatre, and admission could be had for a nickel (fifteen cents if you bought a package deal including trolley fare). Its boathouses and Pavilion can be seen on the left side of the double postcard image below.
A large recreation area was also built at Riverside (on the Weston side of the river) the same year. The Riverside Recreation Grounds included the obligatory boathouse, the largest swimming pool in New England, a football field, a baseball diamond, a track facility and outdoor gymnasium, tennis courts, bowling alleys, a restaurant, a bandstand, and dormitories.
Both venues fast became popular destinations for huge crowds of families and young adults from all over the area.
In 1903, the Metropolitan Parks Commission, which controlled six miles of the Charles River, from Newton to Waltham, issued new rules of conduct for for the crowd of boaters in that area, forbidding activities such as drinking, gambling, and "any obscene or indecent act." From 1903 to 1905, 37 couples were arrested under these guidelines for kissing or lying down in canoes on this stretch of the river.
The young people fought back almost immediately. Canoeists who went out on the river refused to sit up straight, and young women taunted the police on the riverbanks to take action. Late in the summer of 1903, protesters massed below the superintendent's office which overlooked the river.
They "sprawled out" in disregard of MPC rules and set up phonographs outside the superintendent's office playing, as the Post put in, "jesting songs into his ear." Up and down the river, canoeing couples baited the police, playing love songs, throwing kisses at the officers, and disobeying the sit-up rule. 
Local newspapers charged that the problems on the river were all caused by troublemakers from Boston. The former mayor of Waltham surmised that Boston men brought their dates to the river to get them drunk. Yet data shows that of the 37 couples arrested in the two-year period from 1903-1905, only a third were from Boston--the rest were from the suburbs near the river.
In 1904, the MPC police equipped their station with darkroom equipment (the better to do some fast photo development of miscreants) and started monitoring behavior with binoculars. (You can see the MPC boathouse at Norumbega on the right side of the postcard above.) Boathouse owners in the MPC-controlled area cited a 50% loss of business, as many thwarted lovers rode the trains and trolleys to Dedham, which was not yet under the jurisdiction of the Commission.
In 1905, a new summer canoe house and ballroom was built on the banks of the river in Dedham, probably to take advantage of the boom in Dedham canoeing. Called Moseley's on the Charles, its ballroom is still a popular destination for weddings and banquets.
In her diary for June 5, 1905, Cynthia writes:
Helped wash in the morning. In p.m., went up to Dedham canoeing--Pauline, C. Rason, C.H., & I. "Cherries" on way home; Charlie got them for me.
And again on June 15:
Went canoeing this afternoon; it was just lovely on river. Pauline, Charlie, Lizzie, C.H., & I--took lunch.
1905 turns out to have been the pivotal year of change on this issue of behavior on the river. In September of that year, the MPC suffered its first court defeat on one of these indecent behavior charges, and the number of arrests dropped rapidly after that. Between 1906-1910, only four couples were arrested.
According to historian Thomas A. McMullin, whose wonderful article in The New England Quarterly was the source of much of the information in this post:
Scholars have traditionally argued that the shift in American romantic mores occurred in the 1920s, but more recent studies have suggested that the changes began earlier, particularly for the working class.
McMullin attributes these changes to the expansion of white-collar work at the turn of the century, the change in perception of what was acceptable behavior among young women (who had started bicycling and pursuing other outdoor activities, in addition to beginning to date rather than being "called upon"), movies--which showed "abundant images of romantic encounters", and the construction of the trolley network.
Illustration Credits and References
 The quotes above, and much of the other background information in this post, were taken from an article by Thomas A. McMullin in the September, 2000 issue of The New England Quarterly. The article is entitled: "Revolt at Riverside: Victorian Virtue and the Charles River Canoeing Controversy, 1903-1905." McMullin (at least at the time) was an Associate Professor of History at U.Mass. Boston.
The colored postcard at the top of this post is entitled "Canoeing on the Charles River," and it shows a photograph taken in 1905, and probably colorized a few years later. The sepia photo further down is a 1905 double postcard entitled "The Charles River from Weston Bridge, Auburndale, Mass." These postcards are part of a terrific collection of early 20th century postcards of the river that belong to the Newton History Museum. More postcards, as well as additional information on the Charles River in Newton, Norumbega, and the canoeing craze can be found at the Newton History Museum Website.