In this screenshot from a video shared Monday by Capital City Fire/Rescue, the waters of Suicide Basin have topped the low point in the dam holding the reservoir and are beginning to flow downhill. (Screenshot)

In this screenshot from a video shared Monday by Capital City Fire/Rescue, the waters of Suicide Basin have topped the low point in the dam holding the reservoir and are beginning to flow downhill. (Screenshot)

Suicide Basin begins releasing water, but flooding risk unknown

Mendenhall Lake levels are declining, not rising

Suicide Basin began its long-awaited release of water late Monday, but there are no signs of incipient flooding along the banks of the Mendenhall River or Mendenhall Lake.

In fact, water levels within the lake and river system dropped Monday and Tuesday following the end of weekend rainfall.

“That’s telling us there is not that much inflow,” said Aaron Jacobs, a hydrologist with the National Weather Service in Juneau. “We’re not putting out any advisories or warnings or watch statements yet because we’re still seeing how this is going to evolve.”

For the past eight years, Suicide Basin, in a side lobe of the Mendenhall Glacier, has been collecting and releasing water with seasonal regularity, occasionally causing minor flooding along the lake and river that share the glacier’s name.

The worst flooding was in 2016, and the scientists who monitor the basin have previously told the Empire that water levels within its reservoir are at or above 2016 levels.

It’s not yet clear whether that means the Valley should expect another summer of high water.

Since it began causing regular floods, the basin has released its water through channels beneath the glacier. Those channels are normally blocked by ice, but when the water pressure within the basin exceeds the strength of the ice, the channels break open, allowing the basin to drain.

This year is different. In late June, an ice shelf more than 1,000 feet long calved into the basin. It was thought at the time that the titanic tumult might jar the channels open. Instead, the ice appears to have sealed the basin more tightly. With the subglacial channels blocked, the basin has filled to the point that it now tops the dam holding water within the basin.

Late Monday, the basin’s water began flowing over the dam and downhill, along the boundary between the glacier and the eastern rock wall that defines its border.

A helicopter video shared by Capital City Fire/Rescue on Monday (embedded below) shows that flow disappearing into the glacier after traveling several thousand feet downhill. In prior years, it has taken several days for water released from the basin to begin affecting the level of Mendenhall Lake and River. It is not clear whether this different avenue will also take several days to show effects.

In addition, it is not clear how quickly water will be released from the basin via this new channel. The flow of water over the dam is expected to gradually erode it, allowing more water to flow out of the basin.

“It’s too early to tell how this is going to evolve,” Jacobs said.

• Contact reporter James Brooks at or 523-2258.

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