SIEM PANG, Cambodia — Kang Ngan isn’t sure what a dam is. She doesn’t know how old she is, although she thinks “over 30” is a good guess. But she has lived long enough to know that something is terribly wrong along the Sekong River.

The rhythms of life here in Siem Pang, Cambodia, are tightly bound to this sleepy, brown tributary of the Mekong, which swells annually during the monsoon season, then gently spills its banks in October.

But last week, the waters rushed up three months early. Thousands of people in this remote northeastern district were displaced, including Ms. Kang Ngan and her community, who are members of the Kavet minority.

The deluge was caused by the failure of a dam in neighboring Laos, about 50 miles north. The accident devastated a large swath of that country’s Attapeu Province, drawing international media attention, but few noticed when the floodwaters rushed downstream and into Cambodia.