This glacier on Mount Rainier in Washington is far shorter than it was a century ago.

It’s one of thousands in North America that are losing ice as the world warms.

As glaciers shrink, the melting is disrupting habitats for everything from bacteria to fish. Scientists are racing to understand the changes.

When it was built in the early 1900s, the road into Mount Rainier National Park from the west passed near the foot of the Nisqually Glacier, one of the mountain’s longest. Visitors could stop for ice cream at a stand built among the glacial boulders and gaze in awe at the ice.

The ice cream stand near Nisqually Glacier around 1915. Mount Rainier National Park Archives

The ice cream stand is long gone. The glacier now ends more than a mile farther up the mountain.

As surely as they are melting elsewhere around the world, glaciers are disappearing in North America, too.

This great melting will affect ecosystems and the creatures within them, like the salmon that spawn in meltwater streams. This is on top of the effects on the water that billions of people drink, the crops they grow and the energy they need.

Glacier-fed ecosystems are delicately balanced, populated by species that have adapted to the unique conditions of the streams. As glaciers shrink and meltwater eventually declines, changes in water temperature, nutrient content and other characteristics will disrupt those natural communities.

“Lots of these ecosystems have evolved with the glaciers for thousands of years or maybe longer,” said Jon Riedel, a geologist with the National Park Service who has established glacier monitoring programs at Rainier and other parks.

“We’re pulling this thing apart but we don’t really understand the impacts,” Dr. Riedel said.

This is the Herbert Glacier, near Juneau, Alaska.

McGrath ALASKA Tanana Lake Minchumina MOUNT DENALI Nenana Fairbanks Cantwell Talkeetna Cook Inlet Kenai Anchorage Homer Big Delta Seward Whittier ALASKA Gulkana Tanacross Prince William Sound Valdez MOUNT WRANGELL Cordova Nabesna Glacier ALASKA CANADA Gulf of Alaska Bering Glacier MOUNT SAINT ELIAS MOUNT LOGAN Burwash Landing Hubbard Glacier Seward Glacier Yakutat Bay Pacific Ocean Yakutat CANADA Whitehorse MOUNT FAIRWEATHER Carroll Glacier Brady Glacier Skagway Hoonah Herbert Glacier Juneau CANADA Sitka Pacific Ocean Dease Lake LeConte Glacier Wrangell Andrei Glacier ALASKA Hydaburg Ketchikan Dixon Entrance Hecate Strait Prince Rupert Sandspit CANADA Terrace Smithers Burns Lake Bella Bella Queen Charlotte Sound Prince George Port Hardy Klinaklini Glacier Quesnel CANADA Williams Lake Campbell River Courtenay Tofino Powell River Lillooet Nanaimo Pacific Ocean Vancouver Kamloops Abbotsford Revelstoke Victoria Bellingham Salish Sea MOUNT BAKER CANADA Kelowna Penticton Everett Aberdeen Seattle Olympia WASHINGTON Astoria Nelson MOUNT RAINIER Longview Wenatchee OREGON Creston MOUNT ADAMS Portland Sheridan Yakima McGrath ALASKA Tanana Lake Minchumina MOUNT DENALI Nenana Fairbanks Cantwell Talkeetna Cook Inlet Kenai Anchorage Homer Big Delta Seward Whittier ALASKA Gakona Tanacross Prince William Sound Valdez MOUNT WRANGELL Cordova Nabesna Glacier ALASKA CANADA Gulf of Alaska Bering Glacier MOUNT SAINT ELIAS MOUNT LOGAN Burwash Landing Hubbard Glacier Seward Glacier Yakutat Bay Pacific Ocean Yakutat Whitehorse MOUNT FAIRWEATHER Carroll Glacier Brady Glacier Skagway Hoonah Herbert Glacier Juneau CANADA Sitka Pacific Ocean Dease Lake LeConte Glacier Wrangell Andrei Glacier ALASKA Hydaburg Ketchikan Dixon Entrance Prince Rupert Sandspit CANADA Hecate Strait Terrace Smithers Burns Lake Bella Bella Queen Charlotte Sound Prince George Port Hardy Klinaklini Glacier Quesnel Williams Lake Campbell River Courtenay Tofino Powell River Lillooet Nanaimo Pacific Ocean Vancouver Kamloops Abbotsford Revelstoke Victoria Bellingham Salish Sea CANADA MOUNT BAKER Kelowna Penticton Everett Aberdeen Seattle Olympia WASHINGTON Astoria Nelson MOUNT RAINIER Longview Wenatchee Creston OREGON MOUNT ADAMS Portland Sheridan Yakima McGrath ALASKA Lake Minchumina MOUNT DENALI Nenana Fairbanks Cantwell Talkeetna Cook Inlet Kenai Anchorage Homer Big Delta ALASKA Seward Whittier Gakona Prince William Sound Tanacross Valdez MOUNT WRANGELL Cordova Nabesna Glacier Gulf of Alaska Bering Glacier CANADA MOUNT SAINT ELIAS MOUNT LOGAN Burwash Landing Hubbard Glacier Seward Glacier Yakutat Pacific Ocean Whitehorse MOUNT FAIRWEATHER Carroll Glacier Brady Glacier Skagway Hoonah Herbert Glacier Juneau Sitka CANADA Pacific Ocean Dease Lake LeConte Glacier Wrangell Andrei Glacier ALASKA Hydaburg Ketchikan Dixon Entrance Prince Rupert Sandspit Hecate Strait Terrace Smithers Burns Lake Bella Bella Queen Charlotte Sound Prince George Port Hardy Klinaklini Glacier Quesnel Williams Lake Campbell River Courtenay Tofino Lillooet Nanaimo Pacific Ocean Vancouver Kamloops Abbotsford Revelstoke Victoria CANADA Salish Sea MOUNT BAKER Kelowna Penticton Everett Aberdeen Seattle Olympia WASHINGTON Astoria Nelson MOUNT RAINIER Longview Wenatchee Creston MOUNT ADAMS Portland Sheridan Yakima ALASKA MOUNT DENALI Fairbanks Cantwell Talkeetna Kenai Anchorage Big Delta ALASKA Seward Whittier Gakona Tanacross Valdez MOUNT WRANGELL Cordova Nabesna Glacier Bering Glacier Gulf of Alaska CANADA MOUNT LOGAN Burwash Landing Hubbard Glacier Seward Glacier Yakutat Whitehorse MOUNT FAIRWEATHER Carroll Glacier Brady Glacier Skagway Pacific Ocean Herbert Glacier Hoonah Juneau Sitka Dease Lake LeConte Glacier Andrei Glacier ALASKA Hydaburg CANADA Ketchikan Dixon Entrance Prince Rupert Sandspit Terrace Hecate Strait Smithers Burns Lake Bella Bella Queen Charlotte Sound Prince George CANADA Port Hardy Klinaklini Glacier Quesnel Williams Lake Campbell River Courtenay Tofino Lillooet Nanaimo Pacific Ocean Vancouver Kamloops CANADA Abbotsford Victoria MOUNT BAKER Kelowna Penticton Aberdeen Seattle Olympia Astoria WASHINGTON Nelson MOUNT RAINIER Longview Wenatchee Creston MOUNT ADAMS Sheridan Yakima Portland ALASKA MOUNT DENALI Kenai Anchorage Big Delta Seward Tanacross Valdez MOUNT WRANGELL Gulf of Alaska MOUNT LOGAN Yakutat CANADA Skagway Juneau Sitka Pacific Ocean Ketchikan Prince Rupert Smithers Burns Lake Bella Bella CANADA Port Hardy Quesnel Campbell River Tofino Vancouver Kamloops Penticton Seattle WASHINGTON MOUNT RAINIER Portland It’s one of thousands of glaciers in Alaska. They are losing about 75 billion tons of ice each year. British Columbia has a string of glaciers along its coast. They are losing mass, too, as the climate changes. In Washington, the glaciers are fewer and farther between.

As a glacier like the Herbert retreats, it leaves an altered landscape. The water that runs through the landscape changes as well. Researchers want to know how the changes may affect what organisms thrive there.

J. Ryan Bellmore, a biologist with the United States Forest Service in Juneau, studies freshwater food webs, the complex what-eats-what relationships that show, in effect, how energy moves through an ecosystem.

Often that involves examining the stomach contents of fish.

One day late last fall, Dr. Bellmore and a colleague set fish traps in the Herbert River, which meanders for a few miles from the glacier to its outlet on the coast.

Sitting on a snow-covered gravel bank the next day, Dr. Bellmore and two other researchers, Allison Bidlack and Adelaide Johnson, were collecting data on their still-live catch — about 50 salmon and similar fish like steelhead trout, all juveniles.

They weighed and measured each fish.

Then, they performed a delicate task called gastric lavage. By carefully sticking the thin tip of a plastic syringe down the throat of each fish and injecting water, Dr. Bellmore was able to flush the stomachs of their contents. The food, nearly intact flies and lots of unrecognizable detritus, was put in bags for thorough inspection and cataloging later. (The fish were eventually returned to the river, apparently no worse for the wear.)

Dr. Bellmore wants to know how what fish eat in the Herbert, a river that consists almost entirely of glacial meltwater, compares to what they eat in other nearby rivers where the flow comes from rain runoff, snowmelt or groundwater. Water temperatures differ depending on the source, as do concentrations and types of nutrients and the clarity or cloudiness of the water. All of that has a bearing on the type of food available and the overall health of the ecosystem.

Studying food webs in different kinds of streams will lead to a better understanding of what will happen as glaciers like the Herbert continue to retreat and meltwater flows eventually decline, replaced by water from other sources.

“As glaciers recede, glacial streams may look more like snowmelt streams, or rainfall streams,” Dr. Bellmore said.

“What unique characteristics are going to go away when we lose these glacial streams?”

One thing that may be lost are specialized communities of organisms.

Streams that are mostly fed by glacial meltwater often have unique species that have adapted to the cold conditions. Reducing or eventually eliminating the contribution of this meltwater will raise stream temperatures. Even a small temperature increase can have potentially negative effects.

“Certain species like cold water,” said Alexander M. Milner, a professor of river ecosystems at the University of Birmingham, in England, who has studied the changes wrought by shrinking glaciers for years.

“When the glaciers disappear, those species will go extinct,” he said.

Dr. Milner was talking about small creatures like insects or even micro-organisms like bacteria.

The impact on larger species like salmon and other fish may be more complex — and, in the Pacific Northwest and Alaska, of more pressing concern. “In this part of the world salmon are just really important for economies and cultures,” said Jonathan W. Moore, an ecologist at Simon Fraser University in Burnaby, British Columbia. With other researchers in the United States and Canada, Dr. Moore has formed a group to more closely study the links between salmon and melting ice.

Salmon and similar fish are born in freshwater streams, then head to sea to grow, often for years, before returning to fresh water to spawn and die. Water temperatures must be just right for salmon to develop, so, as glacial meltwater declines and stream temperatures rise, populations may be affected.

But other changes may be beneficial, at least for a while.

As glaciers retreat, they leave boulders, gravel and sediment, and a stream running around them. The gravel beds and meanders can be ideal places for salmon to lay their eggs during spawning season.

In some places there may be huge gains in salmon habitat, said Kara Pitman, a Simon Fraser doctoral student in the group who is modeling how and where habitat increases as glaciers shrink. In some areas of British Columbia, she said, “in 25 or 50 years, watersheds may be full of salmon.”

But over all, the gains might not be so great, given the other changes affecting the fish. “It’s more that salmon populations could be redistributing,” colonizing new areas and abandoning others, she said. “Salmon do have an amazing ability to adjust and adapt.”

“We’re trying to understand the new opportunities for salmon,” Dr. Moore said, “but fully acknowledge that climate change over all is not good for salmon in most places.”

Glacial retreat causes changes over years or decades. But shrinking glaciers can also lead to floods or debris flows. These can alter streambeds in an instant, as they’ve done here on Tahoma Creek at Mount Rainier.

Slowly or quickly, the loss of glaciers is disrupting the landscape, at Rainier and elsewhere.

The natural history of the Pacific Northwest and Alaska, and of other parts of the world with glaciers, can be seen as a story of disruption caused by ice.

It happened in the past, during the last ice age, when the movement of the huge ice sheets that covered much of the planet reshaped the land.

And it’s happening today, as glaciers, the remnants of those ice sheets, retreat.