For the first time in 13 years, threatened freshwater mussels are reproducing.
Conservationists have been keeping an eye on the River Irt, which supports a population of about 300 "critically endangered" mussels.
There are now very few freshwater shellfish populations in England.
Chris West from West Cumbria Rivers Trust said it was "thrilling" and "heartening" to find evidence the species was reproducing. .
Populations have been affected by wildlife crime, habitat deterioration and declining water quality.
It is also not helped by the reproduction process which sees females release millions of glochidia, or larvae, into the water which have to be inhaled by fish to survive.
The glochidia clamp on to the gills of juvenile salmon or trout for about a year before landing on gravel where they bury and grow.
The process is called encystment and the last evidence of it happening in the River Irt, in the Wasdale Valley, was recorded by the Environment Agency in 2010.
To try and help the mussels, conservationists have been improving the habitat of the river by stabilising the river bank, planting willow and trees and adding boulders to the water.
They also released juvenile mussels, which had been bred in captivity, to help increase numbers. .
"It's thrilling to find evidence that the population is reproducing. All our previous monitoring has found no evidence of reproduction, so this is heartening to see," Mr West said.
"It's also reassuring to see the mussels released into the Irt are thriving. These plus more that we plan to release over the coming years will significantly boost the population number. ".
