Looks like Miscanthus sinensis. Some varieties are definitely hardy to zone 4.

Here is a link that might be useful: Miscanthus Seedheads

Thank you, that does look like what I have and several varieties are hardy to zone 4 like you said.

The only thought is that miscanthus is a cultivated and not a wild grass. If you didn't get it from someone's garden, then I would assume that it is a wild grass, like a phragmites (reed grass), that can be invasive. It would probably be hardy in your area.

Here is a link that might be useful: phragmites common reed grass

It certainly could be phragmites. xpress is right that miscanthus would not likely be a wild grass in zone 4.

If it is phragmites, you'd see it growing in a more spread out pattern along the water...not so much clumping. You don't want to harvest/plant phragmites :)

Here is a link that might be useful:

Hmmm.... just doesn't look like a Phragmites bloom to me.
If it was growing in a 'wild' area it is more than likely Miscanthus saccharifloris. A very hardy Miscanthus often seen in ditches & other 'un-cultivated areas' It makes a large patch as is grows in a 'running' fashion rather than clumping like Miscanthus sinensis cultivars.

As for Miscanthus sinensis in 'wild areas' I think we are going to see more & more of this grass turning up in areas where it can compete with the native flora. I am seeing more & more Miscanthus seedlings turning up in my gardens. Our autumn weather has been warmer & the frosts much later so the Miscanthus cultivars are actually ripening good seed in zone 5. There is a lovely batch of self-sown M. sinensis growing along our road below a hillside planting - the self-sown plants are growing out of a 'tar & chip' road surface!

Here is a link that might be useful: miscanthus saccharifloris