"There are a large number of feed lots being operated directly adjacent to the Rangitikei River which would have held many tens of thousands of cattle over winter," Mr Carlyon said.
"The impacts of the feed lot activity are significant and long lasting.
"The loss of sediment to the river is evident but there is very real concern at both the nutrient and pathogen loads entering the Rangitikei River in an uncontrolled manner."
Following The Catalyst Group's release of aerial photos Horizons councillor Rachel Keedwell raised the issue at Wednesday's Environment Committee meeting.
She wanted to know what the council's role was in regulating the practice.
"I would suggest the education approach may not be working when you look at the scale (of) these feed lots," she said.
Regulatory manager Nic Peet said the One Plan didn't provide a way to deal with feed lots.
"At the moment I would agree that the One Plan is relatively silent in terms of rules and policies - so that's a plan change issue," Dr Peet said.
"However, if any activity be it feed lot or otherwise is causing a significant environmental effect then the Resource Management Act (RMA) does enable us to take action."
Dr Keedwell said there needed to be monitoring to asses whether there was a significant environmental impact.
"If we've got concentrated mob of cattle, the effluent they're producing is equivalent to a small town and where saying we don't have to regulate?
"Is a short staffing issue? Is it a decision that we don't want to go down that road?"
Dr Peet said the council could adopt more stringent rules via a plan change.
"Otherwise it's essentially a case of someone causing a significant environmental effect as a result of what they're doing that we can identify."
Chief executive Michael McCartney said the issue would likely be discussed as part of a future One Plan review.
But Mr Carlyon said the onus should not be on the community to prove an adverse effect and believed the RMA made it "quite clear that the activity should not occur unless it was consented".