Why is the information available to archaeologist about early native american culture is very limited?

The primary reason for the limited information available to archaeologist about early Native American culture is their inability to record their history in tangible ways like artifacts or remains of the houses etc; instead they passed on the information orally. Their houses etc were made of mud or wood which by time were affected by biodegrading and nothing was left behind. 

what are local authorities doing to fix the problem

explanation:

answer; i believe the correct answer is (the willamete valley);