The Environment Agency team was inspecting a weir to see if it is stopping fish from migrating further up the river.
Salmon and sea trout have been seen in the River Aire at Knostrop, just downstream of Leeds.
Weirs can stop fish getting further upstream to breed.
Some are suitable for alterations that can make journeys easier for fish.
The inspection was to check whether the Leeds weir would be suitable for a fish pass. If it, is, agency officials will have to find the money for its construction.
Fisheries officer Pete Turner said: “Water quality in the River Aire is now good enough to sustain salmon and sea trout, but we have a legacy of structures which stop them getting upstream to breed.”
EU measures have been introduced that take other factors into account, such as the potential to support wildlife and fish movement.
Migrating fish, such as salmon, sea trout and eels, are finding it difficult to get up the rivers Calder and Aire because they are blocked by 40 weirs.
The passage of fish is among standards rivers have to meet to be considered healthy.