The Troutdale

This semi-confined (Transmissivity = 20,000 gpd/ft, Storage Coefficient = 2 x 10 minus 3) aquifer is so "Robust".  That word comes to mind, given the recent local increases in use in the Gervais and Lake Labish areas.  The irrigation "work horse", cemented Pliocene-aged sand and gravel, is covered by a 30 to 50 foot thickness of Quaternary-aged silt (The Willamette Silt).  Fifty percent of annual precipitation enters through the silt and re-fills the Troutdale.

Geological cross-sections and maps show some features of the Troutdale Formation and the overlying and underlying formations.  The Troutdale is relatively flat, being deposited in wide channels and in smaller-scale lobes.   There is much more structure in the older Oligocene geology (Eugene Formation) and in the Miocene Columbia River Basalt.  The present comments concern locations near Mount Angel and the strike-slip Mount Angel Fault.  The sand and gravel is thicker near the Pliocene outlet, at the town of Turner, Oregon, yet twelve miles further south of the south side of this cross-section. The intervening Mount Angel fault seems to affect a reduction in the sand and gravel abundance as observation proceeds northward in this horizontally unscaled figure.  The wells are municipal wells on the secondary highway.

Sand channels in the Troutdale Formation extent from wider sand and gravel areas near and south of the City of Woodburn.

I am very impressed with signs of powerful tectonic activity that once occurred on the Mount Angel Fault, before the deposition of the Willamette Silt.  Near fault deposits were angular blocks of basalt.  The heave on the fault is more than 500 feet.

A pointed butte appears near the commercial truck stop at Aurora, Oregon.  This is LaButte.  I just had to extend a project cross-section through the area of this figure to investigate its origin.  Deep wells by able well drillers were chosen in this study, to investigate the geological contacts.

This picture is also helpful in explaining the annual rise and fall of the water table.  When late fall and winter rains come, the water level builds up into the Willamette Silt.  The high water table discharges by gravity into deeply incised channels in the land surface, dwindling to "base flow" whereby creeks have pools but flow is very still.  Main channels, such as the Pudding River (Pudding as in "Chocolate" as in much brown mud in the water), have a good rate of flow.   The static level is hydraulically connected to the Troutdale below the Silt and that water level changes from well above to just above the contact of the two Formations.

The flat-lying deposition of the Troutdale is apparent in the above figure as is the contrasting structure of the basalt, where it is folded locally into an angular unconformity.  Otherwise, the contact is disconformable.

Back to water in the Troutdale:  Water levels, this year are down only about a foot since last year's outstandingly high static water level.  In the Gervais area the level is 25 feet and in Labish it is 50 feet.