Depending on size of creek?If there is room build the wetland the side then build a drain system from the creek to the pond if the creek flows year round.If small creek and small drainage area and flow drys duringsummer the you have to store water in off season for later water source.
If flows year around i would use only for water source for your wetland.
As for material for your water system.Could use dual wall drainage tile like Chris uses in his projects.Agdrain as custom built flow box.You could build you own out of channel iron and stop logs(tongue and groove boards)
I have build a few wetlands and was goverment cost share.We used steel pipe the main pipe went straight throw the damso you could drain the wetland down if you wanted too.There was a larger stand pipe with stop logs in it where you could adjust the water level(ad or remove logs,boards) water flowed over the boards and out the bottom pipe.
I would consider the wetlnd option over too the side if creek and drainage area is very large.or you will be having to rebuild or repair thing during flood seasom.Good luck.