The Zion Narrows closed last weekend for the first time in 2012; the hike is open again now, but will certainly close a few more times throughout spring. Since we are pretty big Narrows Nerds here at ZAC, I thought it might be fun to provide some “big picture” background info for those eager to know if The Narrows and other technical canyons will be closed during YOUR visit.
Like all things in The Nature, a wide variety of factors influence Spring flooding. But for the most part, spring water volume is determined by three primary factors:
Snowpack – The snowpack on the Markagunt Plateau (elevation 8000 ft.) north of Zion feeds the Virgin River as it melts, contributing significant water volume to the North Fork watershed each spring. One year ago today, the snowpack was 112″, while today, the snowpack is only 54″. That’s a big difference. Drive up to Cedar Breaks along Highway 14, and you can see the snowpack for yourself.
Daily High Temperatures – When temperatures are really warm, you can almost SEE the snow pack shriveling on the Plateau. Moderate temperatures, however, melt snow slowly, causing a slower, more gradual melt. If you watch the USGS river gauge chart, you’ll see water that melts every afternoon (hottest part of the day) takes about 8 – 10 hours to reach the Virgin River gauge, which peaks every day around midnight. At night, temperatures cool down again and snow stops melting… 8 – 10 hours later (morning), river levels are much lower. Since the National Park Service uses the 24-hour high mark to decide Narrows closures, there are many days when the river is actually under 150 CFS, but closed due to exceeding this boundary the previous evening.
Precipitation – Obviously, if colder weather brings snow to the Markagunt Plateau, that means the snow pack grows and high water levels will last longer. If warmer weather causes RAIN instead of snow, however, the rain will drastically reduce the snow pack, quickening the spring melt.
Each year, we find ourselves watching the USGS river level data during spring runoff, scrutinizing the USGS water flow charts to predict if/when The Narrows will close and open. 2007 and 2011 are great examples of the wide runoff dichotomy that The Narrows can display. In 2007 (the green line on the chart), the winter snowpack was quite low; the river barely surpassed 150 CFS, and The Narrows was only closed for a few days. In 2011 (the magenta line on the chart), however, the snow pack was roughly double that of 2007, and The Narrows was closed well into July. So what will 2012 be like?
As you can see from the snowpack charts, 2012 is very similar to 2007. In fact, until the snowstorm on March 18/19 increased the snow depth by 50% (!), the snowpacks were nearly identical. Based on this data, the running hypothesis around here is The Narrows might be closed off-and-on for a few weeks to a month, but we are generally expecting a great spring, with The Narrows open for most of March, April, and May.