Well, here's a possibility. The population of Douglas County did not actually grow steadily between 1858 and 1870; from 1862 to 1863 it probably shrank slightly. This was ultimately due to the way a large part of the county land was acquired from its previous owners through the Treaty of Traverse des Sioux in 1851. Under the Treaty, the Sioux handed over all their land in Minnesota, except for a reservation along both sides of the Minnesota River, in return for a cash payment and other benefits, but they soon found that the cash wasn't enough to purchase necessities which they could not obtain within their reservation, so in 1858 they handed over the reservation land north of the river, in return for a great increase in the original Treaty payment. However, they were paid in instalments, and were also at the mercy of official supply agents. Within a few years, a combination of late payments and corruption by the agents led to a situation where the supply warehouses were full, but the Sioux were starving. A reckless crime by five young men, in August 1862, forced their leaders to make an all-or-nothing decision, and (knowing that Government troops were busy fighting the Confederate States) they declared war, attempting to reclaim the land in the absence of the contracted payments. As indicated by the pale colour on the map, this included a large part of Douglas County, including the area which would later become Solem, and the adjoining settlement of Holmes City. The few settlers there fled to Alexandria, which had not been Sioux land, and also had a measure of protection against attack. Despite the shortage of Government troops, the uprising was swiftly suppressed, and in 1863 a new treaty was created, forcing the Sioux to leave Minnesota altogether. Gradually, places like Holmes City were re-established, and after the end of the Civil War, veterans were ancouraged to take up new land on the frontier. Exiled in Dakota, the rightful owners of the land were further outraged when gold was discovered within their reservation in the Black Hills, and prospectors started moving in while the government turned a blind eye. In 1865 a new war began, under Red Cloud, and despite the availability of battle-hardened troops after the Civil War, the government was unable to control the situation. Despite (or perhaps because of) this, settlement in Douglas County accelerated. The County was formally constituted in 1866, as were some of its earliest communities, including Alexandria and Holmes City. This seems to have been the spur for large-scale immigration, both of US citizens from further east, and those many Swedish and Norwegian settlers who would appear in the 1870 census. [A short acount of one such migration can be seen, at the time of writing, on the Archived version of a reunion website, based on the diary of pioneer Engebret Lobeck from Norway, who found on arrival at Holmes City in May 1867 that "there were several Norwegians and Swedes together with Yankees who had taken most of the finest land"]. So just what would all these people do if Red Cloud's warriors were able to cause so much trouble that the government was forced to take notice of their grievances?