“It just keeps rising,” he said.

The geology is complex, but it boils down to this: Relieved of billions of tons of glacial weight, the land has risen much as a cushion regains its shape after someone gets up from a couch. The land is ascending so fast that the rising seas  a ubiquitous byproduct of global warming  cannot keep pace. As a result, the relative sea level is falling, at a rate “among the highest ever recorded,” according to a 2007 report by a panel of experts convened by Mayor Bruce Botelho of Juneau.

Image At low tide, it is possible (but messy) to hike across exposed mudflats around Juneau. Credit... Michael Penn for The New York Times

Greenland and a few other places have experienced similar effects from widespread glacial melting that began more than 200 years ago, geologists say. But, they say, the effects are more noticeable in and near Juneau, where most glaciers are retreating 30 feet a year or more.

As a result, the region faces unusual environmental challenges. As the sea level falls relative to the land, water tables fall, too, and streams and wetlands dry out. Land is emerging from the water to replace the lost wetlands, shifting property boundaries and causing people to argue about who owns the acreage and how it should be used. And meltwater carries the sediment scoured long ago by the glaciers to the coast, where it clouds the water and silts up once-navigable channels.

A few decades ago, large boats could sail regularly along Gastineau Channel between Downtown Juneau and Douglas Island, to Auke Bay, a port about 10 miles to the northwest. Today, much of the channel is exposed mudflat at low tide. “There is so much sediment coming in from the Mendenhall Glacier and the rivers  it has basically silted in,” said Bruce Molnia, a geologist at the United States Geological Survey who studies Alaskan glaciers.

Already, people can wade across the channel at low tide  or race across it, as they do in the Mendenhall Mud Run. At low tide, the navigation buoys rest on mud.