Gurdy, you are so very welcome!
My zone is right on the edge of change 6a, 6b, & 5.
I'm a bit familiar with the area as my daughter used to live a mile from the lake in Illinois. Try the zip code zone finder. You may also find it a bit warmer since you are in a hill. Warm air rises and the valleys tend to frost and be colder then up the sides of a hill, especially when facing the sun. You also have the rocks to hold in the heat and generate it out at night. So you might be able to 'cheat' a zone near the stone.
Before we start, Michigan has a Landowner Incentive Program (LIP) that you might qualify for.
Im looking for plants to plant along the stream course...
I would definitely consider natives. Here is a [url=https://www.michigan.gov/dnr/0,1607,7-153-10370_12146_12209---,00.html]list of native grasses and sedges[/url].
From that list I would recommend these for along the stream.
Cotton Grass - Eriophorum virginicum will do well in a boggy area that tends to stay wet, if you have [url=https://calphotos.berkeley.edu/cgi/img_query?stat=BROWSE_IMG&query_src=photos_browseimgs_plant_sci&where-genre=Plant&where-taxon=Eriophorum+virginicum]such an area[/url].
Little Bluestem - Schizachyrium scoparium will grow in nice clumps and be easier to control.
Junegrass - Koeleria macrantha shouldn't overtake the area.
Switchgrass - Panicum virgatum makes great cover and food for ducks and small mammals.
Tussock sedge - Carex stricta will grow in nice clumps in or out of the water. The second site has others I would recommend or have recommended EXCEPT Iris versicolor - blueflag iris. That can take over the pond, your neighbor's yard and on. I had some in a pot in the ground and it split the pot and kept going!
Pennsylvania sedge - Carex pensylvanica will creep and form a colony.
Liverwort - Conocephalum conicum doesn't bloom and you can probably find some on the forest floor to transplant near your stream in a moist shady spot. It would look wonderful growing on the rocks where you now have algae. The female plants will get an interesting umbrella-like stalk.
Jack-in-the-pulpit - Arisaema triphyllum
The Jacks above look nice interplanted with Marsh marigold - Caltha palustris.
For your pond any of these natives would be just fine. Do be certain to keep the Golden Club in a container as it can be a bit invasive in the mud.
I would like some type of perenials to plant along the top of the wall that may drape flowers over the wall.
Geranium 'Rozanne' is one of the longest blooming hardy perennials I can think of. If you plant it near the edge of the wall it will drape over. It can take full to part sun and blooms in my daughter's zone 6/7 garden from June to hard frost. It's not a native geranium but it's not invasive. You can see it spilling into a walk in the first picture and over a pot in the second pic. It will grow to about 3' wide in just a couple of years if it's happy.
There are many sedums, some that grow upright and others that are a great groudcover that will grow over the wall and even root into the spaces between the rocks. [url=https://www.ces.ncsu.edu/depts/hort/consumer/factsheets/groundcover/sedum_sieboldii.html]Each plant only spreads about 1' to 2'[/url].
Arabis caucasica - rock cress is not native but will grow in and around the rocks and cascade over a bit. The second pic is without the flowers.
Bearberry aka Kinnikinnick - Arctostaphlos uva-ursi is an evergreen native that will grow about 10" tall and 3' to 5' wide and cascade over the wall. It offers berries in the winter too.
Gaultheria procumbens - wintergreen is another wonderful native evergreen shrubby groundcover that will cascade over the wall and feed wildlife. If you look closely at the last link you will see it over a wall.
Mitchella repens - partridge berry is another woody evergreen native with berries but will need some shade if you have a shady spot. It [url=https://www.ct-bohttp://departments.bloomu.edu/biology/ricketts/Mitchella/M_repe/M_repe.htmltanical-society.org/galleries/mitchellarepe.html]roots along the ground[/url].
...also some sort of plant that might work as a good anchor point to the left or both sides of the catch basin where the large waterfall comes over the top wall.(the green spot)
Do you want a small tree such as a weeping Japanese maple, a shrub or an herbaceous perennial?
Algea has gotten extreemly unbearable this last summer.
Consider barley straw for algae control.
Once you finish your 'homework'
get back to me and I'll see what I can come up with for the top of the wall at the waterfall.