A month after severe March flooding swept the Midwest, the Missouri River is still running high, complicating recovery for many communities in the region. Sections of Scott Olson’s farm near Tekamah, Neb., are still underwater. Once the water goes down, Mr. Olson, 62, said he has much work to prepare his fields for planting before the end of June, after which it might be too late. Still, he considers himself lucky compared with other farmers in the area who lost their homes, grain storage, livestock and machinery. …