The Bulgaria, operated by a local tour company and marketed to Russians, sunk Sunday in 65 feet of water about two miles from shore.
According to the Wall Street Journal, about 58 remain missing, including dozens of children. About 80 survivors were rescued by a passing ship, while others were retrieved by rescue workers or scrambled to the river's edge on makeshift rafts.
The New York Times is reporting that there were at least 59 children onboard -- most of which have not been rescued -- and that 36 of those children were listed as having the same birthday, suggesting that forms were filled out improperly. The exact number of children that were onboard remains unknown.
A spokeswoman for Russian Emergency Situations Ministry, speaking on Russian TV, said that the chances of finding more survivors was unlikely.
The ship went down at 2 p.m. local time (6 a.m. EDT) near the village of Syukeyevo in the Tatarstan region while on its way to Kazan, about 450 miles east of Moscow. The cause is unknown, though it was reportedly raining heavily at the time of the incident.
According to the Associated Press, a spokesman for the regional Emergencies Ministry told the state news agency RIA Novosti that numerous survivors have said the ship was leaning to starboard during a turn and a wave washed over the deck. And a survivor who spoke to a Russian TV station said the boat "tilted to the right and sank within minutes."
Russian President Dmitry Medvedev has called for an investigation of the tragedy and for a full inspection of the country's other passenger vessels, said the AP.
The Bulgaria was built in the 1950s in what was once Czechoslovakia. Though the vessel is said to have had dozens of cabins and two restaurants, the ship was most likely on a day-long outing on the Volga, a popular summertime pasttime for Russians.
This is the peak season for tourism on the Volga, which is lined by many of Russia's largest cities.
--by John Deiner, Managing Editor, and Jamey Bergman, Web Content Producer