I am not completely sure, but my guess would be that this Latvian postcard shows Venta Rapid, a rapid on the Venta River in Kuldīga. It is the widest rapid in Europe – 249 metres and up to 270 metres during spring floods It also features the widest waterfall in Europe,110 m wide, but only two metres high. In the stone, canals are carved. They were used to catch salmon and sturgeon. The fish that didn't make the jump up the river were swept by the current back into the canals where they were caught in hanging baskets.
Many thanks to Oleska!