Its a good discussion, and I’m interested in the context of the study of intermittent streams.
There are a lot of examples of detecting “water leaks”, that is the physical situation of going from no water, to “tap water” – a relatively low TDS water.
For an intermittent stream, I imagine its going from a physical water present (with potentially high TDS) to a water drying up condition. That is the sensors are likely to have the full “benefit” of living water, algae – and then gradual drying up.
Where it gets placed in the stream channel and how “wires” are run out is part of the setup. I would think a sacrificial outer cord, flexible PVC tubing, would be basic starting point. Some critters won’t like the woven metal outer shields of some cables.
For low power I would think that would also support a periodic check – ie a test every 15minutes. That may help with the powering scenario.
Working with indoor tap “water leak” detection I bought a water alarm with a “4ft rope connector” Honeywell RCHWES4/U RCHWES4 Wi-Fi Water Leak Detector Rope Sensor https://www.amazon.com/gp/product/B01J7EVB1G/ref=ppx_yo_dt_b_asin_title_o07_s01?ie=UTF8&psc=1
I haven’t experimented with the way that it works yet :). I just did the basic test of wet parts of it and see the alarm go off, before I installed it under the floorboards, directly beneath a bath where I had some water leak in the past.
So it may take a few small steps to find out what works best in the field.