Riverfront land and riverfront homes are so desirable and unique that it is really difficult to assign value or provide specific pricing guidelines. Each riverfront location has merit regarding views, terrain, power and road access. In most cases you will need more than one acre. Anything less than 3 acres may require a basket load of cash and a lot of engineering to meet building standards in El Dorado County, and may feel a bit cramped.
Most riverfront property lines in this area extend to the center line of the river. The survey map in this post shows an actual two-acre parcel on The South Fork of the American River. Of this two acres, 1.3 acres is underwater at typical summer flows. When the river flooded in 1997, the entire parcel was underwater. So you may need more acreage than you realize!
For a prime riverfront home site — about 5 acres in an estate setting — you may have to pay over a million dollars to convince owners to part with their rare jewel.