1) The dam at N42°03.660', W92°55.212' is the Center Street Dam, apparently an old concrete low head dam that may have been used to generate ice for cutting in the winter, according to an older resident. At around 800 cfs, the dam is almost entirely submerged,as in the accompanying photos. It looks as though it would be a problem causer at extremely low flows, as there is a fairly large eddy pool on river right below the dam. There are no warning signs, but there is a possible portage on river left (at least at 800 cfs and lower.)
2) The Sanitary district "recently" constructed a rock dam at N42°04.138', W92°53.706'. At flows of lower than 800 cfs, it will not be navigable due to shallow passage. At higher flows, it likely will not be dangerous because of (at this point in time) shallow waters below the dam -- no way for a recirculating eddy to develop with only 2' of water depth. Whether that will deepen as time goes on, no way to tell without seeing the engineering data.
At 800 cfs and higher, there is a good portage on river right. At lower flows, an additional portage opens up on river left.
Upstream there are signs in two spots warning of the "low head dam." The closer of the two is shown on the accompanying map.