As John mentions, it is nearly impossible to locate vacant land
without also locating some trash. His language for contract to purchase is great.
One other way to get the creek cleaned up is to contact the local
authorities about the situation and see if they have any way to get a volunteer
day set up for cleaning up the creek.
Many times asking the question can result in media attention that results in people
coming to help with cleaning up areas such as the creek you mention.
Good luck and remember, there are lots of great folks around that are willing to give of their time and effort for a good cause. Local authorities may offer help in the way of machinery to haul off the trash collected.
Some times you can get help from the State Fish and Game or Forestry offices, it just depends on their take of the problem and how they are approached.