Riverine data in some form have been collected by all civilizations, so essential are rivers to commerce and agriculture. Records of the annual high-water level of the Nile, for example, are complete all the way back to A.D. 622, save for one large gap in the early modern period. The U.S. government in the mid-1800s began attempting to gather reliable meteorological information on its rapidly expanding and geographically diverse domains, and by the turn of the century runoff data in the form of hydrographs existed for many of the important western rivers. It occurred to a number of investigators that if, by means of a crude model, one could correlate, year after year, the size of the snowpack at the moment of its greatest extent—the moment of what is now called "ripeness"—with the streamflow, then one would have a powerful forecasting tool. Moreover, if one measured the snowpack not only at the moment of greatest extent, which usually occurs in April, but also in March, February, and January, and kept detailed annual records, one might even be able, eventually, to make a preliminary forecast as early as midwinter, based on past trends. Of course, even if this methodology worked to perfection, it would never reveal how much snow had actually fallen or how much water that snow actually contained. It could, however, reveal that this year's runoff had a certain probability of being, say, roughly 20 percent less than average, or 15 percent more.
That, at any rate, was the idea entertained at the turn of the century by, among others, James E. Church, an active outdoorsman who was a professor of Latin and Greek at the University of Nevada, in Reno. Church was animated in part by the situation of Lake Tahoe, which epitomized the competition for water in the West. In this case the competition involved farmers downstream, who needed the lake's water to irrigate their crops; a hydroelectric-power company, which operated several power plants on the Truckee River, whose source is Lake Tahoe; property owners on the lake, who wanted its level to remain constant; and the Paiute Indians, who by federal treaty were guaranteed a certain amount of Tahoe water to feed their Pyramid Lake. Being able to predict snowmelt would simplify the management of the lake.
Because evaluating water content was his aim, and because the water content of snow varies widely from storm to storm and even within the context of a single "weather event," Church contended that a simple depth measurement would probably not suffice. The old rule of thumb for snow-to-water conversion is that ten inches of "average" newly fallen snow make for about one inch of water, but some snow is much wetter and some is much drier. In Colorado, famous among skiers for its powder, twenty inches of snow or more may be needed to produce one inch of water. The samples that Church required in order to create a historical index had to reflect the water content, the "snow-water equivalent." Church therefore employed a stainless-steel cylinder, which he forced through the snow to extract samples from the surface of the snow to the ground. Once obtained, the samples would be weighed, weight being a reliable index of water content. Church's extraction instrument, which was more than six feet long, and onto which extensions could be screwed, came to be known as the Mount Rose sampler, after the mountain where he conducted his first surveys. Subsequent models have been waxed, polished, baked with silicon, and provided with teeth, and the stainless steel has given way to aluminum, but their design remains based on the original.