Are you sure the surname was Hickson? Almost all of the Census info I found is Hinkson.
In 1850 Aldana and Gratia were living in Rumford, Oxford Co., ME.
Aldana Hinkson, b. ME, 30, farmer.
Gracia A., b. ME, 30.
Loring K., b. ME, 4.
Elizabeth M., b. ME, 3.
Charles F., b. ME, 1.
In 1870 Aldana, Gratia and Loren were living in West Bear River, Yuba Co., CA.
Aldney Hinkson, b. ME, 51, farmer.
Ann Gracia, b. ME, 50.
Loran Gracia, b. ME, 25, farmer.
In 1880 the family was living in Greenwood, El Dorado Co., CA as you previously stated and the name on the Census looks like Heinkson.
Rachel and Rosa must have died or Rachel and Loren divorced after the 1880 but before the 1887 Spokane Census.
In 1887 Aldana, Gratia and Loren were living in Spokane Co., WA.
Hinkson, A D, male, b. ME, 66, farmer.
G. A., b. ME, 63, female.
L K, b. ME, male, 38, ME, farmer.
In 1900 Loren was living with his new wife Joanna and her children from a previous marriage in Asotin, Asotin Co., WA. They had been married four years, therefore, they were married about 1896 either in Spokane or Asotin Co.
In 1910 Loren and Anna were living in Asotin Co., WA.
You could contact the Asotin Co. Clerk, 509-243-2060 and ask if they have the marriage record on file.
You can contact the Asotin County Historical Society, contact info at www.museumsusa.org/museums/info/1157692 and see if they have the Asotin County Directories for the years the family lived there. If they do, ask if they'll copy the pages for your family for a donation.
In 1920 Loren and Johanna Hinkson were living The Dalles, Wasco Co, OR, as you already know.
In 1930 they were in Asotin again with the last name of Hinkson.
Loren K., b. ME, marr @ 32, 84, no occupation.
Anna J., b. MO, 75, marr @ 24.
A Loren K. Hinkson, Age-90, died 11 Sept. 1935, Asotin, Asotin Co., WA, Father Name=Loren Hinkson [probably index mistake], Mother=Gracia, Spouse=Jo Anna Degge. Source: Death Index, www.digitalarchives.wa.gov/ . Successful Searching!