River, which rises in Tibet and provides irrigation water to Pakistan from rapidly retreating Tibetan glaciers, and the Colorado River, which receives much of its water from seasonal snowpack in the Rocky Mountains and provides irrigation water to some 4 million acres (1.6 million hectares). Structures Snow is an important consideration for loads on structures. To address these, European countries employ Eurocode 1: Actions on structures - Part 1-3: General actions - Snow loads. In North America, ASCE Minimum Design Loads for Buildings and Other Structures gives guidance on snow loads. Both standards employ methods that translate maximum expected ground snow loads onto design loads for roofs. Roofs Snow loads and icings are two principal issues for roofs. Snow loads are related to the climate in which a structure is sited. Icings are usually a result of the building or structure generating heat that melts the snow that is on it. Snow loads – The Minimum Design Loads for Buildings and Other Structures gives guidance on how to translate the following factors into roof snow loads: Ground snow loads Exposure of the roof Thermal properties of the roof Shape of the roof Drifting Importance of the building It gives tables for ground snow loads by region and a methodology for computing ground snow loads that may vary with elevation from nearby, measured values. The Eurocode 1 uses similar methodologies, starting with ground snow loads that are tabulated for portions of Europe. Icings – Roofs must also be designed to avoid ice dams, which result from meltwater running under the snow on the roof and freezing at the eave. Ice dams on roofs form when accumulated snow on a sloping roof melts and flows down the roof, under the insulating blanket of snow, until it reaches below freezing temperature air, typically at the eaves. When the meltwater reaches the freezing air, ice accumulates, forming a dam, and snow that melts later cannot drain properly through the dam. Ice dams may result in damaged building materials or in damage or injury when the ice dam falls off or from attempts to remove ice dams. The melting results from heat passing through the roof under the highly insulating layer of snow. Utility lines In areas with trees, utility distribution lines on poles are less susceptible to snow loads than they are subject to damage from trees falling on them, felled by heavy, wet