Spring fed runoff

I don’t know what the proper term for this is. I assume it’s not really a pond. This is our natural spring shed runoff. How should I maintain this? The second photo is what it looked like yesterday, picture this says it’s water cress. But it was so thick you could see no water and it was impeding the flow down to the pond that the previous home owner dug out. There are some rocks lining this but not enough. How can I stop this water cress and other plants from essentially overtaking it ? Do I need to lay anything inside the water ? More gravel ? I would rather not use anything plastic or toxic. Today I weed wacked what I could then dug out the sludge with my hands for now.