Debris flows carrying saturated solid materials in water flowing downslopes often cause severe damage to the lives and properties in their path. Close monitoring and early warning are imperative to save lives and reduce damage. Current debris-flow-monitoring systems usually install sensor equipment along the riverbanks and mountain slopes to detect debris flows and track their data. Unfortunately, most of this equipment indirectly collects data only from a distance. So far, there is no way to understand what is happening inside a debris flow and to collect its internal parameters, not to mention doing this in real time. To answer this challenge, this paper presents a novel multifunctional wireless sensor for monitoring debris flows. The core idea is to let these sensors drift with the debris flow, to collect flow information as they move along, and to wirelessly transmit the collected data to base stations in real time. The design of such a sensor needs to address many challenging issues, including cost, deployment efforts, long-term standby, and fast reaction. This paper addresses these issues and reports our evaluation results.