Quick translation of part from an article of journal “Ochrana Přírody” describing restoration of degradaded riverarm just by conecting it to the river. <link> to the whole article in czech.

An interesting location where the river branch was reconnected and where lateral fish migrations are currently being monitored is the Doubka (Felinky) pond in the Ostrá cadastral area. For decades, the pond was separated from the Elbe River by an artificial earth embankment, the purpose of which was supposedly to protect fish from the then heavily polluted water from the Elbe. However, the separation of the pond from the river persisted even after the water quality in the river had improved significantly over the years. As a result of preventing water exchange between the river and the pond, negative phenomena associated with it gradually began to manifest themselves - in particular, significant eutrophication of the pond (primarily due to fertilizer runoff from the surrounding intensively farmed fields, wastewater from the cottage settlement, leaf litter from the surrounding trees), resulting in regular, turbulent development of cyanobacteria in the second part of the growing season. This caused regular oxygen deficits in the pond. Insufficient oxygenation of the water in combination with the inability of lateral migration of fish between the pool and the river resulted in repeated massive fish deaths at the site (Daněk et al. 2023). These environmental changes were reflected in a change in the composition of the fish community, with the most numerous species being those resistant to oxygen deficiency (e.g. the invasive silver crucian carp – Carrasius gibelio) and the most rapidly reproducing species – the invasive non-native eastern minnow (Pseudorasbora parva) comprising up to 80% of all individuals in the fish community. The pool was reconnected in March 2021, using a 20 m long underground culvert (height 2 m, width 2.5 m, water column about 180 cm), which overcomes the earthen embankment with the road and allows for the exchange of a significant amount of water between the river and the pool during fluctuations in the water level, as well as bilateral lateral migration of fish. Since the beginning of the connection, fish migrations have been monitored using a pair of reading frames placed in the culvert, which record all fish marked with an RFID chip. The connection of the pond with the river had an admirable effect in this case, when, firstly, thanks to the water exchange, long-term oxygen deficits in the pond were no longer recorded, and then there was an extreme transformation of the fish community, when the previous community dominated by the non-native eastern minnow was recorded after only two years, a community dominated by native species - small bream (Blicca bjoerkna) and common roach (Rutilus ruttilus), with a high incidence of predatory species, many of which - e.g. asp (Leuciscus aspius), European perch (Perca fluviatilis) - were not originally recorded in the pond at all. Monitoring of chipped fish with reading frames in the culvert showed that the culvert is widely used by fish (hundreds of recorded migrating individuals) and some individuals even migrate between the river and the pond on a daily basis, when e.g. An individual catfish (Silurus glanis) measuring 85 cm in length regularly migrated from the river to the pool after dark and then from the pool back to the river before dawn.