A monitoring system includes one or more monitoring devices, positioned in sewer manholes, storm drains, etc., and a remote monitoring station that communicates wirelessly therewith. The monitoring device may be an integrated unit, including sensors, a two-way telemetry unit, a power supply, a processor, and supporting hardware, all located in an enclosed, waterproof housing. The monitoring device is placed within a manhole cavity to obtain depth (e.g., water level) measurements, images, and other data, and report the measurements back to the remote monitoring station, which analyzes the data and responds to alert messages when a dangerous water level is detected. An additional sensor may monitor the manhole cover for security purposes. A distributed mesh network of wireless nodes may be used to relay communications from the monitoring devices along alternative paths, through bridge nodes that may connect to a public wireless or cellular network.