I had an instance where the city shut of the connection from the storm sewer to the waste sewer and a heavy downpour one afternoon caused the storm drains to overflow the sewer system and rise up and flood my basement about 8 inches. I reported it to FEMA and they told me this was not a flood and to sue the city for damages. I did and was awarded damages because the storm sewers are supposed to overflow into the waste sewers first, it helps washout the waste sewers and prevents flooding like I experienced.
BTW, in Michigan there is no other flood insurance offered outside of FEMA insurance, I’ve talked to and confirmed this with numerous agents.
As I mentioned, the HO policy excludes “flood, surface water, waves, tidal water, overflow of a body of water . . .”. The creek qualifies as surface water because it’s on the surface of the planet. Also, please do not overlook the sequence description of, “We do not insure for loss caused directly or indirectly by any of the following. Such loss is excluded regardless of any other cause or event contributing concurrently or in any sequence to the loss.”
There is also an exclusion for earth movement which would include erosion. The underwriters of HO policies did not want to have an exposure to this type of loss. In order to argue for coverage under the above scenario, both the surface water and erosion exclusions would have to be overcome. Personally, I could not hope to win that argument.
And that’s really the whole point since the OP is actually trying to decide on whether or not to purchase this home. People all over live next to creeks and trees, better to be pro-active and remove ones that pose a danger to the home instead of waiting for a strong wind to come by and see what the insurance covers. Also, from what I’ve observed trees along a creek generally end up falling into the creek since that is the side that gets eroded.