An ice dam along a river is caused by large changes in temperature in winter. When the temperature gets very cold for several days, ice builds up on the sides of the river. When temperatures rise, rather than melting, the ice breaks off and flows downstream. At some point, it may clog the river as in these photos. Two of the images show an ice dam in a river in Goffstown and New Boston, NH. The last image shows the river as it normally looks. This ice dam is about a mile long. It causes water to back up and may cause flooding in some nearby homes. It will clear when the spring temperatures and rain melt the ice enough that it breaks up.