The river in Purgatory that restores the memory of good deeds is Eunoe. At the summit of Mount Purgatory, souls who have completed their purification drink from Eunoe after passing through Lethe, so the memory and strength of their virtuous actions can return.
The two rivers serve complementary purposes in Dante's Earthly Paradise. Matelda immerses Dante in Lethe, whose waters erase the memory of sin. She then leads him to Eunoe, whose waters strengthen his recollection of the good he accomplished. The soul is therefore prepared to ascend to Heaven with both burdens removed and virtues fully remembered.
Eunoe is one of Dante's inventions rather than a standard river from classical mythology. Its name is generally connected with Greek roots meaning something like “good mind” or “good understanding.” Dante mentions its sweet water at the end of Purgatorio, in Canto XXXIII, the cantica's final canto.
Lethe is the most likely wrong answer because it is also a river at the top of Mount Purgatory. However, Lethe removes memories of sin; Eunoe restores or reinforces memories of good deeds.