Derek Arnold found a seam.

With less than two minutes remaining in the second period, the UMass Lowell hockey forward found himself all alone near the blue line with no opposition in sight, and with defenders stacked to one end of the ice in a clear breakdown, teammate Joe Houk rifled a pass to him streaking in front of the visitors’ bench.

Arnold had the rest covered. With nothing but clear ice separating him and Massachusetts goaltender Kevin Boyle, he skated in and backhanded a shot by the net minder to give his team a decisive two-goal lead heading into the intermission.

And it was just that kind of night for the Minutemen, who committed mistake after mistake that ultimately led to their demise in a 6-3 loss to the No. 14 River Hawks in front of 3,124 at the Mullins Center on Friday night.

“I think all of their goals were results of very poor decisions (by us),” UMass coach John Micheletto said. “I don’t know if I have an answer for that. I just know that it was uncharacteristically poor reads that led directly to their goals.”

It was those kinds of mistakes compounded by them being early and late in periods that made the night so frustrating for the Minutemen as well. Four of UML’s six goals on the night occurred in either the first or last two minutes of a period, including two from Arnold, both of which came after UMass cut its deficit to one goal.

“It’s very difficult to generate energy, feel good mentality when that happens,” Micheletto said. “When things happen in the first couple of minutes of a game or the last couple of minutes of the period, it takes that wind out of your sails. Now you’re having to manufacture energy and confidence at that point.”

Arnold’s first strike came late in the first period. After the River Hawks opened up a 2-0 lead, including the first coming just 1 minute, 20 seconds following the opening faceoff, UMass forward Michael Pereira sliced his team’s deficit to one with a short-handed wrap-around goal at the 16:54 mark that seemingly was going to give the Minutemen momentum heading into intermission.

But two minutes, nine seconds later, UML found itself on a 2-on-1 attack down the ice and Houk found Arnold, who put it past Boyle to give his team a 3-1 lead.

UMass forward Conor Sheary, who was stoned by River Hawks goaltender Connor Hellebuyck on a breakaway opportunity in the first, put home a power play goal at the 9:38 mark of the second to make it a one score game.

But again, Arnold responded, as he streaked down the right side unmarked and deposited it past Boyle to give UML a 4-2 lead heading into the second intermission.

“I really like the fact that we responded,” River Hawks coach Norm Bazin said. “It’s tough on the road, especially when the crowd gets going and there’s a hostile environment in some respects, to respond after the opposition scores are very good signs.”

Rocco Carzo tipped in a shot from Joel Hanley at the 7:10 mark of the third to bring the UMass deficit back to one, but again, UML responded. Riley Wetmore scored his 100th career point by putting home a rebound with 4:56 left, and 2:53 later, Josh Holmstrom put the icing on the cake with a power play goal to make it 6-3.

“You give up two power play goals and a shorty, usually you don’t win those games,” Bazin said. “Thankfully for us, we scored a power play goal of our own and we were able to score 5-on-5.”

Boyle finished with just 13 saves while Hellebuyck recorded 26 saves in the win.

The loss puts the Minutemen in eighth place in the Hockey East standings after Vermont leapfrogged them with a 2-1 win at Northeastern on Friday night.

UMass will try to avoid a season sweep from UML when it heads to the Tsongas Center on Saturday night to complete the weekend’s home-and-home series.

Stephen Hewitt can be reached at [email protected] and followed on Twitter @steve_hewitt.