There are a lot more waterways suitable for something like a 415 than airports that are suitable for a 747 or DC10. From the Bombadier site:
Werbil.
Also from the Bombardier website.....
How much distance does the Bombardier 415 take to scoop its load? The
Bombardier 415 is very manoeuvrable.
If a body of water is 1,341 metres (4,400 feet) long by 90 metres (300 feet) wide and 2 metres (6 feet) deep, without floating debris, then it is scoopable*. Only 400 metres (1,350 feet) are actually required on the water, the remainder being needed for approach and climb-out. Of course, these distances can be reduced by
scooping partial loads or scooping while turning. Speed on the water while scooping is 75 knots. Approximately 10 to 12 seconds are required between touchdown and lift-off to scoop a load.
* Including
obstacle clearances of 50 feet, sea level, on a standard day, all operating engines
As I lives adjacent to the upper Murray, I know how rare this specification is anywhere other than the lower 1/4 of the Murray. Our rivers are narrow, wind almost continuously, are full of snags and tend to be lined with Eucalypts well over 50' in height. Not to mention the ambient temperatures on a typical bad fire day.
The specifications could be met on a handful of major impoundments, but as our lakes are man-made and contain much flooded timber, safety considerations would require the aircraft to only scoop from carefully surveyed and marked runs.
There may be more such runs than there are airports capable of taking a 747 , but not a lot of them.
Respectfully............ Peter