The former head of fisheries management for Alberta is calling for an independent inquiry after he says a gravel pit which flooded west of Edmonton three years ago led to a drastic reduction in fish stock in the North Saskatchewan River.

Duane Radford said the trouble began when a flood in June 2013 cut a side channel through the Dahm gravel pit, located upstream of Devon near the Genesee power plant. The pit is operated by Mixcor Aggregates Inc., and the flood buried large pieces of the company's equipment and a fuel tank for several months.

Berms built in the fall of that year to stop the river from breaching the pit failed. Construction of a fifth berm was completed in January 2014. Radford, a fisheries scientist, said sediment levels in the water exceeded compliance limits for almost two weeks during the berm's construction.

The sediment disturbed and added to the river by supposedly dirty materials through the construction of the project has resulted in close to a 50 per cent decline in the fish population below the Dahm pit, Radford said. The sediment smothers fish eggs and damages gills of the adult fish, he said.

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A high hoe is submerged in the river after it flooded through the gravel pit in 2013. (Supplied)

"It's a huge berm, probably one and a half times the length of a football field, so literally trainloads of sediment would have washed in the river downstream of the site," Radford said. "It appears as though a tremendous amount of sediment … was added to the river. And sediment is a death warrant for fish. That would have had a major impact on the fish, particularly the young fish."

Keith Rae owns Get Hooked Fishing Adventures and has fished the river for 20 years. He said the river downstream from the gravel pit looked like chocolate milk during the construction of the berms, and that a sediment plume was visible for several miles downstream. He brought the issue to Radford's attention in 2013.

Rae completes yearly fish surveys. He first noticed a big decline in 2013, when he caught and released 1,917 fish compared to 2,851 the year before. He noticed a further decline in 2014, when he netted 1,305.

The area around and below the gravel pit are where the most extreme drop in fish numbers were noted in Rae's count. Fish numbers upstream remained consistent, he said.

It was after analyzing this data that the problem became "obvious," Radford said.

Radford said he has contacted his MLA about the issue but has not gotten a response. He now wants an independent inquiry done into the issue, because with other gravel mines along the river, he said it's a problem that he doesn't want to see again.

The men are in the process of contacting Fisheries and Oceans Canada, the province and the city of Edmonton, and will be bringing their concerns to members of the Alberta Fisheries Management Round Table.

"I hope this would be a wakeup call to the gravel mining industry, the government, and regulators," Radford said.

"This sort of event could happen again if steps aren't taken beforehand to minimize the risk and if it does happen again, what's going to be done? Are we going to go through this again? That's got to be totally unacceptable."

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A fuel tank sits on its side at the Mixcor Dahm gravel pit near Edmonton after a flood in 2013. (Supplied)