What a lucky person to have an intermittent creek as part of their landscape. Are there restrictions, buffer zones attached to this creek...even though it is on your property I am pretty sure it's regulated with riparian restrictions. First thing you do is look in your mortgage documents for legal stuff, call your city, county for their input...then find out what is happening upstream. Are there golf courses upstream, Monsanto, dumps...does it flow through other yards? Do those residents understand and abide the rules?
I'd assume the water flowing through your yard is unsafe to drink for sure, I'd probably have the water tested. On the positive side there might be incentives available to you as well as money, grants and riparian professionals to help you, money to have rip-rapping (erosion control with rock), plant costs and even plants provided.
Since you have a dry creek bed part of the time, I'd definitely be thinking about boulders, river rock. When it is dry, are you able to irrigate your plants while they get established? I don't know Pennsylvania at all, I am familiar with the Pacific Northwest. Your zone is...similar. My recommendations are: Salix purpurea 'nana' (my all time favorite skeleton...plants that tie your entire landscape together cohesively... shrub and you'd need to shear it 2x a year to look similar to an umbrella/funnel? Shear the top flat, then round it towards the ground so that the edge is wider than the top...like an umbrella. Still, very naturalistic looking but if you don't shear the shrub, even though it has a 'nana' tag can grow 20X20 and look rangy. Blue gray tiny foliage and cinnamon stems that move in the wind. Stems are thin, easy to prune and they 'glow' cinnamon in the winter); Carex elata 'Bowles Golden' will grow in or out of water; a perennial that might work well is Ligularia dentata 'Othello' and Ligularia stenocephala 'The Rocket'...both would add great texture, color, height...you'll have to make sure they stay watered. Only the sedge I would plant in the water...does fine when creek is dry but you will have to water. Ligularia and the Blue Arctic Willow...keep to the margins of the creek, not in it. Keep the number of TYPES of plants to a maximum of 4-5. Powerful views need framing! Consider using a grove-type tree such as Amelanchier alnifolia, Serviceberry, with multiple stems and plant 3 or 5 or more. It is a small tree, 4 season beautiful, few pests, but not to be used IN the creek. Or consider one or three of Salix mansudana tortuosa 'Scarlet Curls'...prune to thin and aerate for light shade, can handle water and poor soils.
Send a picture and let me know what you find out about the legal stuff on your creek! Lucky, lucky you!