Geology at the Colville River Mouth - Lake Roosevelt, WA

Colville River Mouth. The Colville River enters the Columbia River at Rivermile 100 just south of Kettle Falls, WA. Its an easy hike along its north bank out to the point if the water level is low and your footing is sure. March is typically a low-water month. Park at the pullout near the Hwy 25 bridge, carefully cross the bridge (narrow shoulder), and descend a fisherman's path down to the rocky flat along the river. Traverse west towards the sandy outcrops seen in the distance. Stay out of the mud. Out and back is less than 2 miles. Turn around if you reach the broad grassy flat where the forest and bluffs disappear. Other shoreline exposures can be seen across the inlet, below the highway guardrails. Those are not part of this hike, but can be accessed during low water by hiking east from the Rest Area or by dropping down through the trees from wide pullouts along the highway.

Hiking map. Park near the bridge and hike the beach for a mile or so, then turn around and go back. Reward yourself at a local brewery.

Cemented and uncemented clastic dikes. A large, sheeted clastic dike follows what appears to be an older, vertical joint that has been cemented with calcium carbonate (CaCO3). This unusual sandstone fin stands proud of the outcrop, while the uncemented dike next to it recedes with erosion and occupies a shallow trough. The fin forms the left margin of the dike. It appears to be contiguous with a cemented horizon near the top of the exposure that continues horizontally off to the left (west). A grainsize change up there must restrict water movement a bit, causing minerals to precipitate and harden here. You can see small 'diving boards' of cemented 'sandstone' casting shadows in the upper left part of the photo. Short horizontal thumbs protrude from the left side of the fin, suggesting cementation tried to follow bedding at several horizons. Carbonate-rich bedrock nearby is likely providing the cementing agent.The fin is not fault gouge and no evidence of movement or shearing is seen along its trend. In fact, horizontal stratification mimicked in the surrounding sediments is well preserved and clearly visible. The large clastic dike follows the pre-existing weakness, as is common for many sand dikes penetrating basalt in Columbia Basin. The interesting thing about this and other large dikes nearby is their width and location. I've argued for years that sheeted dikes in Eastern Washington are products of floodwater loading and hydraulic fracture, not earthquakes. These dikes are located 285 km from the Wallula Fault Zone, one of the more active faults in the Yakima Fold Belt. There are no significant Quaternary faults recognized nearby. Because of the remote location, we can confidently say these dikes have nothing to do with seismic shaking generated by faults in south-central Washington.

Standing proud. The cemented fin casts a shadow.

Sheeted, wedge-shaped dikes. Several Touchet-type clastic dikes cut the sandy section. They are uncemented, pinch in the downward direction, and are truncated at their tops by colluvium and mass wasting deposits. About a dozen are present in the bluffs here, ranging from 2-50cm wide.

Boulder lithologies. I counted at least a dozen different rock types along the beach. Porphrytic andesite (Eocene?), white quartzite (Addy?), cemented conglomerate (angular clasts in coarse sand matrix), foliated granitics, bright white unfoliated hornblende granite, purple quartzite (Belt?), amphibolite(?), weathered granite, weathered basalt, minor CRB basalt, vein quartz, and young sandstone concretions.

Sandstone concretions. Cannonballs, snowmen, turtles, baguettes are just some of the shapes taken by sandstone concretions weathering out of bluffs composed. The material is glacial sand, which is curious. Cementation here affects horizontal beds and vertical dikes. Cementation suggests either considerable age (pre-late Wisconsin sand?) or a local groundwater source containing high levels of dissolved CaCO3. Unclear what that source is.

Graffiti wall. Look for concretions at the base of the graffiti wall. They collect there, but weather out from a bed higher up.

Look closely at the chunks. Some of the large chunks of broken, cemented 'sandstone' in this slide deposit are cut by clastic dikes. Diking preceded sliding. Thick sands sure look like they were rapidly deposited. Sliding looks fairly young. Everything here is within the Ice Age floodway.

Avoid the mud. Wet areas on the flats are miserable to hike across. Stick to the gravel.

Backfilled burrows. Mazama ash high in the section is riddled with backfilled rodent and insect burrows. The ash is well exposed in a low shoreline bluff at the west end of the hiking route. Its thickness and white-pink hue are unmistakable.

Easy hike for the sure-footed. Park at the forested terrace pullout seen at left. Cross the roadway bridge and hike the shoreline at right, rounding the bend in the distance and continuing west for roughly a mile.