I see this having only two elements:

Remote sensor assembly at the riverbank. This will need to be robust for the conditions but cheap enough to be replaceable if it gets swept away!

Probably some kind of float or optical sensor to detect the level, with arduino and RFM69-type transmitter to signal the home unit.

Power will have to be solar + LiPO.

RFM69 as this has the range required. Signalling will be pretty simple and low bw.

Arduino as I have most experience with this platform.

Home Base unit. Should be pretty simple with radio-link receiver and microcontroller to activate the alert.

Maybe add a remote-alert option (SMS/email/etc) later for when no-one is home.