There are different rules for boat ramps and boat docks. For a boat dock you need to contact the soils conservation district. They will tell you all their requirements for a dock....it cannot be a permanent structure, it ahs to be removed in the winter, etc....They also have an application and a permit that you have to file for. The boat ramp is another story. Those are permitted thru the soils conservation district and the army corp of engineers. There is a dual application and permitting process to go through for those. I have the name and number of a local professional that can lead you through either process.
High voltage power lines do lower the value of your property. Those powerlines are in an easement that crosses your property. There are things that you can and cannot do on that easement. It is best to contact the company, I believe it is Bonniville Power, to obtain a list of their regulations.
The Clark Fork River has flooded its banks,. but I an not sure of the exact location of the flood. There are flood plain maps that you can use as a guide to see if you are in the flood plain. Please remember these are only guides. To determine if you are in the floodplain, you have to shoot elevations of the flood plains, your property and your home. The same professional that can answer your questions about the docks can also help with the flood plain information. Please give me a call and I will pass along his information.