 Underscoring the storm and flood threats gripping much of the upper Midwest, a large portion of a house that was teetering on the edge of the Blue Earth River in southern Minnesota collapsed into the water yesterday. It was near a dam that so far is still holding despite days of heavy rainfall and rising river levels across the region. Separately, the National Weather Service has said it has been assessing damage from several suspected tornadoes reported yesterday in Iowa and Nebraska.