The Environment Agency said investigations suggest the recent burst of heavy rainfall, together with low river flows and high temperatures, has led to a reduction in oxygen levels. This has killed dozens of fish in the river, which has been dubbed by passersby and wild swimmers as "heartbreaking". An Environment Agency spokesman said: "We continue to work with Anglian Water to monitor water quality in the River Wensum in Norwich after fish died in recent days. "Our investigation suggests the most likely cause was a burst of heavy rainfall last week that flushed contamination from drains into the river. "This coincided with low river flows and high temperatures, significantly reducing the amount of oxygen in the water the fish need to survive.