660 OREGON AND NORTHERN CALIFORNIA. 
they form broad plains on each shore. Hast of the Cascade Range, be- 
yond Wallawalla, alluvial plains are said to be comparatively un- 
common. As the rivers flow in a deep bed, usually several hundred 
feet below the table-land above, they only wash the sides of the 
enclosing gorge during freshets. Yet small flats, half a mile wide, 
not unfrequently border the river, even in these deep channels. 
The alluvial tracts along the rivers of the coast section, have been 
described as lying in two separate plains, styled the upper and lower 
prairies. We proceed to give some particulars respecting the extent 
and features of these river flats. 
Willammet Plains to the Mission Settlement.—I first observed the 
upper terrace of the Willammet Valley, below the Willammet Falls, 
fifteen or twenty miles from the Columbia. The forests of the shore 
occasionally opened, and exposed to view from the river, a steeply 
sloping bank forty feet in height. Going up the stream, the terrace 
continued along, varying in its distance from half a mile toa mile. 
The banks of the river, in this part of its course, are twelve feet high 
at low water. ‘Twenty miles up the river, the shores were rocky ; yet 
even here, the terraces were often distinct. The upper of the two layers 
of basalt bordering the river, retreats in an undulating line, from a few 
rods to half a mile from the stream, leaving small flats, like bays or 
coves, bounded by a steep, rocky wall, fifteen to twenty feet high. 
Above the Falls to Champooig, a distance of twenty miles, (ascended 
by the writer in a canoe,) the high, wooded bank of the river shut out 
of sight the terrace which is said to exist on the east shore. At Cham- 
pooig, we passed up the bank, (here twenty-five feet high, M, fig. 1,) 
and for a mile inland, travelled over an alluvial plain, the proper 
SECTION OF THE ALLUVIAL PLAIN OF THE WILLAMMET. 
bottom-land ; thence we made a rapid rise of fifty feet, which brought 
us to an upper prairie, the height of which was estimated to be in no 
part over sixty feet above the lower plain, or eighty-five above the 
river level. We continued on the high prairie for twelve miles, cross- 
ing a great bend in the river, and then by a steep path descended 
fifty-five or sixty feet to a small valley, which soon opened upon the 
