The Erra River lower course traverses the Uhaku karst field that disappears under ground in the karst basin known as Suurhaud. The river flows under ground for about 400 m, and emerges in springs at the foot of Purtse high bank. The Pikkhaud karst valley in Uhaku karst field – about 250 m long, 10-30 m wide and 2.5-6 m deep – was formed by the uniting of the chains of the karst cones.

The Uhaku karst field is also an example of the devastating effect of industry on the waters of karst systems. Erra River carries the refuse liquids from Kiviõli to the karst field that form a solid bituminous layer of refuse.