During Russian Civil War the Northern Dvina had great importance for opposing sides. In 1918-1919 the Red, White and British flotillas were very active providing effective and often decisive artillery support for their troops and fighting enemy flotillas.
In RUS Kotlas is the only Northern Dvina port left for Red flotilla after Arkhangelsk captured by Whites and Allies (Brits). It makes the flotilla position very vulnerable and directly contradicts history. In fact though the Northern Dvina flotilla was organized and based in Kotlas (because it had rail connection with Petrograd) it had another convenient shelter. It was Velikiy Ustyug where all flotilla vessels spent Winter of 1918-1919 and from where they sailed to rival Brits in Spring.
My source for the information posted above is "Civil War. Military actions on sea, river and lake systems", vol. II. North West.
I hope that adding a port in Velikiy Ustyug and making navigabe a part of the Upper Norhern Dvina between Velikiy Ustyug and Kotlas will answer history and help to secure the position of Red flotilla.