Srinagar, Aug. 27 -- The nightmare returned as the Jhelum swept into Kashmir's markets, traders threw up sandbags, and families rushed upstairs. In Jammu, a bridge collapsed, and in the Chenab Valley, cloudbursts cut off villages.

The scenes carried the same dread as September 2014, when floods submerged nearly a third of Srinagar and displaced half a million people.

Ten years on, the valley remains exposed to the same forces that once drowned it.

Hydrologists estimate that the Jhelum's carrying capacity has fallen by nearly a third over the past five decades. Silt has raised the riverbed by up to four feet in places, forcing floodwaters over embankments even during moderate rains.

From Sangam to Srinagar, once-broad channels have nar...