Peel carrots; cut into 4-inch lengths, then into scant 1/4-inch thick julienne strips. Cut celery and leeks same size. Set aside on baking sheet.
In large shallow Dutch oven, heat oil over medium heat; cook shallots for 1 minute. Add carrots, celery and leeks; cook, stirring occasionally, until leeks are softened, about 3 minutes. Pour in wine; bring to boil. Boil until most of the wine has evaporated. Add boiling water, and dill and parsley sprigs; return to boil. Reduce heat and simmer, uncovered, for 10 minutes.
Push vegetables to side of pan; place salmon in centre, skin side up. Spread vegetables back over salmon in even layer. Poach, covered, until fish flakes easily when tested with fork, about 10 minutes. With spotted spatula, transfer salmon to half of paper towel–lined baking sheet. With slotted spoon or tongs, transfer vegetables to other side of pan. Remove salmon skin. Cover pan and keep warm.
Meanwhile, remove dill and parsley sprigs from poaching liquid. Bring liquid to boil; boil until reduced to about 1/2 cup, 2 to 3 minutes. Remove from heat. In small bowl, whisk flour with butter until smooth; whisk into poaching liquid until smooth. Whisk in chopped dill, salt and pepper.
Arrange vegetables on warmed plates; top each with 2 pieces of salmon. Drizzle with 1 tbsp of the sauce.