Josh Manson scored twice and had two assists, Brock Nelson had two goals and an assist, Cale Makar had a goal and two assists, and Martin Necas had a goal and an assist for the Avalanche (32-4-7), who scored six times in the second period. Brent Burns scored a goal, Ross Colton had three assists, and Scott Wedgewood made 29 saves.
Shane Pinto and Brady Tkachuk each scored a goal for the Senators (20-18-5), who have lost three straight and five of their past seven (2-5-0). Leevi Merilainen allowed three goals on 12 shots before being replaced by Mads Sogaard at 2:35 of the second period.
Sogaard allowed five goals on 16 shots in his season debut before being replaced by Merilainen to start the third period. Merilainen made an additional six saves.
Manson scored to make it 1-0 Colorado with a one-legged wrist shot from above the left circle that found its way through traffic before going under the leg of Merilainen at 10:11 of the first period.
MacKinnon extended it to 2-0 at 17:14 when he coasted backward to the top of the left circle and fired a wrist shot that went past Merilainen's blocker.
Makar pushed it to 3-0 after MacKinnon dropped a pass to Makar inside the blue line, and the defenseman took it down for the wrist shot from above the left dot at 2:35 of the second period.
Claude Giroux corralled a loose puck at the Ottawa blue line and reversed the other way to Pinto for the rush, who skated in to the top of the left circle and fired a wrist shot that got past Sogaard top shelf on the short side to cut it to 3-1 at 5:08.
Ottawa appeared to have scored to make it 3-2 at 6:15, but Colorado successfully challenged the play for being offside and a video review overturned the goal.
Necas scored a power-play goal at 11:46 to make it 4-1. His one-timer from the bottom of the left circle went short side off Sogaard.
Just 17 seconds later, Burns made it 5-1 after his slap shot from the blue line caromed off Ottawa defenseman Thomas Chabot at the left side of the crease and bounced inside the left post.
Nelson made it 6-1 with a 5-on-3 power-play goal at 14:23. He one-timed MacKinnon’s crossing pass at the bottom of the right circle short side under the glove of Sogaard.
Manson scored his second of the game to make it 7-1 at 16:48 with a slap shot from the blue line getting under the glove of Sogaard, and Nelson made it 8-1 at 18:04 when his one-timer from above the left dot went short side.
Tkachuk scored a short-handed goal at 7:03 of the third period, finishing the rebound of Tyler Kleven’s initial shot through Wedgewood for the 8-2 final.
