Before the actual construction starts on the new bridge an archeological survey of the site must be done. A few dig spots, like this one, are being looked at. You can see the different strata of the soil. Once the survey is done the SDOT website will post data, images and a video of the survey. More information than I can comprehend, although there are some interesting things being found. There were no permanent villages or settlements in this location of the current river and bridge, just seasonal fishing camps.  The black tank holds water they use for sluicing the material as needed.