Hi James
I did my Final year Geography project on the Don, at school , During the project I covered many of the feeders and did an in depth analysis of the water, to find out why it is so prolific when much of the surrounding high ground is Acidic based. We found that there are areas of limestone high in the catchment which contribute a lot of the nutrients to the river, hence the fantastic Ranunculus beds and huge insect population and why it fishes really well in its upper reaches.
When they did the last Don Mangement plan in 2009 they also picked up on this along with Sandstone And Schist which add to the flavour.
"2.2 Geology and Hydrology
The bedrock of the Don catchment is dominated by quartz-based acidic rock types and to
a lesser extent localised areas of granite, both of which are weathering-resistant.
Consequently they introduce relatively low levels of nutrients into the river and offer
little buffering capacity against acidic rainfall.
Some localised areas of the catchment are naturally more nutrient-rich. Basic rock types
such as the gabbro and allied types underlying the Urie and around Strathdon are
important for introducing nutrients to the water, as are areas of Old Red Sandstone
around Kildrummy and the limestone and hornblende-schists upstream of Strathdon.
Typically much of the water chemistry for the Don is reasonably good by northern
Scottish upland standards."
So perhaps not the classic Spring fed Limestone rivers of the south, but very much a limestone infused alkaline chemistry in an area where it is uncommon.
Sandy