My first post was lost in the timeout. All I had saved was the large detailed part about vampires that I decided to cut out before attempting to post.
I've gone through similar although in Nanaimo and Vancouver. And my number one piece of advice is don't feed the vampires!
This first letter in gives no notice of mention of any particular violation. IMO its just to start the psychic dread and fear they feel paid by us to create and feeds them and you will make them very happy if you expose yourself in reply. Normally I always attempted to speak to any such letter writer before getting defensive, but it would seem your letter was written to preclude any quick solution that would be best for the taxpayers (and the waterway if the city actually was concerned about the environment in this case).
Doing it again, I would be clear I will not understand, until they are clear on what in particular they wish to enforce against me. Generally this takes a letter from a contracted Lawyer weeks or months later, but the predators will feed in the mean time off any attempted bargaining, and collect any pleading to use against you should it come to court. With your letter, my only question would be "what legislation requires that I "must" submit you a letter right now? I do not understand."
My guess (never having been to your property) would be they know they have a clear court case for say a 110 foot shed being 10 feet too big for not being permitted, but want to wreck a garden, and want you to offer them something in writing they can use in attempt to wreck a garden. They would need to be masochistic predators to take you to court and go public to kill a historic garden.
If the district wants one to stop work on something, they should put up a stop work order on what they want stopped.
My advice is don't respond until you are back and can see your property in case something was done on it after you last saw it.
On line take a few hours to copy and read the relevant code, don't try to find the angles right now, its too much wasted negative energy, just get familiar with it in case your realize you are rightly in violation of something and you wish to change it (is the mine shaft less than 10 square meters? as example). If not, you neatly file things and try to relax and live life as before the vampires attacked like this.
When back take pictures of your property.
Fix a few simple things (like any 10 square meters restriction) perhaps things like only cedar ties need to be used in the setback zoning? I've not had to research setback zoning, but perhaps a thing or two can improve the garden, for example are there clear restrictions on types of garden bed wood to be used in these areas?
Again try not to dwell (as I did) on the uncertainty and fear of the state saying its angry and will attack your property without telling you exactly why it is angry and what it will attack. Don't feed the vampires with to much excess lingering worry over the weeks or months this may play out until the district finally decides what it wants to predate, has a dull lawyer send you a letter, or has some good faith advice or order for the property.
Then when you get the lawyers letter you finally see their hand, and are in a position to agree to those demands (no court) or fight it.
Good luck Peter. If it seems the city is getting nitpicky about setback gardening, I can show you some horrendous septicfied setback new development destruction they have approved in our area recently.