What is the biggest freshwater lake in the world? A fascinating repeat of the PBS series "Nova" answers the geography quiz tonight. In addition, two recent films made in Baltimore get a cable screening.
* "Nova: Journey to the Sacred Sea" (8 p.m.-9 p.m., MPT, Channels 22, 67) -- The title describes the traditional name for Lake Baikal, which stretches across 400 miles of Siberia and is estimated to hold one-fifth of the fresh water on Earth. The repeat episode examines its ancient history. PBS.
* "Honor Thy Mother" (9 p.m.-11 p.m., WJZ, Channel 13) -- Sharon Gless ("Cagney & Lacey") stars in this TV movie from 1992, as a woman who begins to suspect that her drug-involved teen-age son (William McNamara) and his friends might have killed her husband. CBS.
* "Dateline NBC" (10 p.m.-11 p.m., WBAL, Channel 11) -- Citing statistics showing that more than 200 dams in Georgia failed last year, contributing to serious flood problems, the newsmagazine tries to examine the safety of water impoundments across the country. NBC.
* "The Tonight Show" (11:35 p.m.-12:35 a.m., WBAL, Channel 11) -- Actor David Duchovny, who plays the FBI agent who specializes in odd-but-true cases on the hit Fox series "The X-Files," is a scheduled guest, along with actress Victoria Principal and comedian Jack Gallagher. NBC.
* "Avalon" and "Guarding Tess" (5:45 p.m.-8 p.m. and 8 p.m.-9:35 p.m., Showtime) -- A double feature of on-location-in-Baltimore movies! The first (1990) is director Barry Levinson's sentimental, ultimately remorseful account of immigrant family life over three generations, with Aidan Quinn, Armin Mueller-Stahl and Elizabeth Perkins. Its setting is clearly Baltimore, from rowhouse neighborhoods to The Senator theater. The second picture (1994), filmed in a house in Mount Washington and on location at Loch Raven Reservoir (among other easily recognized locales), ostensibly takes place in Ohio. Shirley MacLaine plays a strong-willed former first lady, and Nicolas Cage plays her Secret Service bodyguard.
* "New Religions: The Cult Question" (10 p.m.-10:30 p.m., MTV) -- Host Kurt Loder and MTV News examine alternative religious organizations attracting young people, from the Church of Scientology, which is followed by Lisa Marie Presley, John Travolta and other celebrities, to a religious community in remote Montana.