The pilot of the tailwheel equipped airplane reported that, while taking off from a gravel bar, as the airplane became airborne over the river, he felt a bump. The passenger reported that they hit something, however the pilot stated that he never say anyone until he continued the climbed and circled over the area, where he noticed a kayaker on the river. He elected to land on a gravel bar near the kayaker, render first aid, and call for medical assistance. The kayaker reported that, while on the Willamette River, she and her mother stopped and rested near the Lower Lamber Bar area, where they observed an airplane on one of the sand bars. After resting, they continued traveling down the river. While paddling in the kayak, she turned towards the sand bar and observed the airplane approaching her. Shortly thereafter, the front tire of the airplane hit her head and overturned the kayak. The kayaker sustained a serious broken leg. The airplane was not damaged. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation.