I don't think it really matters from a stream perspective. Watercress likes moving water of any kind I believe. I am in zone 7b. It freezes but I assume it is much colder where you are. My watercress is severely retarded but not killed in the winter.

Highly doubt watercress would survive your winters but why would you care. I purchase each spring one bunch of watercress at the grocery for 1.50. spread it across the top of my filter which is 4 x 8 ft. By the first of July the filter is covered and over flowing reaching the water 2 foot below to the delight of the goldfish and golden Orfe which love to eat the leaves. As a mater of fact I enjoy it also. Even so I have to trim excess off the sides and back once a week filling a 5 gallon bucket which goes in the compost pile. In the fall I remove it all to the compost bin when I clean out my filter and shut down the system for the winter. A great filtering plant with many side benefits.

Marshmarigolds grow wild up here in Alberta and have successfully wintered for me both with wet and dry conditions. The watercress will come back from seed with luck even after -40 but the plant will not survive. I have it in my spillway which I keep running all winter with a stock tank heater in my pond and it stays green except when it goes below about -30 it springs right back as soon as the temp warms up above -30. I shut my pump off last winter as the spillway was leaking and the watercress was back in the spring from seed. One thing about watercress is that the roots create a massive sheet and can divert the water over the edges if you don't prune them regularly. Great for extra filtration not so much for water levels. Only thing worse that I have found for that is water mint. Water iris is also a super plant for extra filtration i find

thanks! sounds like something I want to do - what is the optimum water depth for cress or marsh marigold - do they need any soil ar all or can you just insert between rocks? Bought some of ea at a nurserhy in pots, plunked them down in their pots in shallow water (feet wet) and soon noticed they werent doing well. The soil in the pots smelled bad=the lovely smell of anaerobic bacteria.

I just stick pieces of watercress under a rock or something. I have never even considered soil. I don't know anything about marsh marigold.

Never considered soil either. Come to think of it, I've never actually seen potted watercress even at a nursery here. I just tuck sprigs from the grocery store under rocks where water flows over the stems.

All I know about marsh marigolds is that they apparently don't care for the climate here. I experimented with them for 3 consecutive years (yeah, I'm stubborn like that) but they refused to cooperate.

Watercress will root while floating around but does better when it is anchored. I find that it blooms best here in the spring when it is cool but will bloom all summer. I plant the marsh marigolds with wet feet in gravel, they can be found around the edges of sloughs around here. They self seed easily and do get a huge smelly rootball over time, the roots are very fine. They are a spring bloomer and are up as soon as the snow cover melts off the top of them. They were blooming while there was still a foot of snow around the pond. They die back for the summer but sometimes rebloom in the fall if the weather is just right.