Wow thanks for the details. Great to see the Sodaq Mbili/GprsBee and hear its good piece of equipment.
When it comes to attaching in the stream be, depends on stream conditions, soil type and vegetation. In the US concrete in the stream channel probably would require a permit – but I’ve seen hydrologist working for the NOAA that will pound a stake into the stream bed, tie it into roots if available. These have been for pressure sensor based equipment. The attached picture is in a channel with the stake pounded into the bed and then a secondary stake tying it into the top of the bank. This was installed in 2007 and flooded to bank height in 2008.