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Avoid Flash Flooding

lightning.pngFlash flooding is the most dramatic and drastic risk involved in desert canyoneering. Every few years, people are killed or seriously injured when they find themselves unaware of and/or unprepared for canyon flooding. Fortunately, we can avoid such tragedies by recognizing conditions with high flooding potential AND having the right knowledge and equipment to identify and mitigate a flooding situation. Flash floods are common during heavy rains, particularly when preceded by either:

1. Excessively dry periods, which bake the dirt of the high plateaus like a clay pot, creating an impervious layer of thick, hard soil. When heavy rains hit this hard crust, the water does not soak into the soil, but instead runs off quickly down-stream to create raging creeks and rivers.

OR

2. After 2-3 days of rain, when the plateau soil becomes saturated and can no longer soak up water. Excess rain then quickly runs into the streams and rivers, sometimes building to dangerous proportions. These heavy rainstorms usually occur in July-September, but it is not uncommon for storms to drop significant rain any month of the year and cause the river to rise very quickly.


The actual "flash," or sudden build up of water, rocks, logs and other debris, may force a wave of water to come rushing through a narrow canyon like Orderville, then junction with excessive run-off in the Wall Street corridor of the Narrows, causing a "roll-through." This roll-thru carries a high density of debris and is deadly. The above images were taken in Zion by a Ranger where Oak Creek Canyon merges with the Virgin River. The accumulated effect of side canyon debris concurrently meeting already rising waters creates a highly dense water flow, unhikable and unswim-able.

Flooding is serious nature. Watch the videos below for a taste of Zion's capacity to change:

Because flash floods are somewhat unpredictable, Zion National Park does not guarantee your safety even on the sunniest day of the year. Occasionally, however, the Park will post closure signs at the Temple of Sinawava to discourage visitors from hiking beyond the end of the Riverside Walk. We can assure you, if the Park has posted the Narrows as CLOSED, do not hike into the river.

Stop by Zion Adventure Company any time for a free flash flood clinic, where we will expand upon the following points:

How to Avoid Flash Flooding

1. Observe Current Conditions Before Hiking

The two primary factors to consider are river level (volume of water in the river) and rain forecast.

1. The US Geological Survey monitors the Virgin River and measures water volume in cubic feet per second (CFS). The CFS of the Virgin River fluctuates over time due to rains and snow melt. Use the chart below to estimate the hike-ability of the river today. Ninety percent of the year the Virgin River flows less than 75 CFS, making it hike-able for any person weighing 50 pounds or more. Zion National Park does not issue Thru-Hike permits when flows exceed 120 CFS, nor do they recommend hiking from the Bottom-Up when flows are greater than 140CFS.

cfscontinuum.png

2. Rain storms can pass through Zion anytime of year. Zion's "monsoon" season, July through September, often features afternoon thunderstorms every-other day or so, while June CAN be bone-dry. Generally speaking, if the forecast calls for a 50% chance of rain or greater, it will likely rain good and hard for at least a little while (and maybe much longer) at sometime during the day. During summer, it is MOST common for rainstorms to move in during afternoon hours; by leaving early, you can often complete your hike before the clouds roll in.

When the rain forecast calls for a 10% - 40%, we often have 20 - 100 people a day come in and ask us, "Is it safe to hike?" The answer is always dependant on the context. We ask ourselves:

• Has it been raining the last few days, or is this the first day for a while?
• Is the river already high (100+ CFS) from snow melt or prior rains? Or is it low and ready to hold much more water?
• Is the soil of the watershed pre-saturated from snowmelt or prior rains, or can it soak up a lot of precipitation?

In any situation where the river is already unusually high AND there is rain in the forecast, we recommend AGAINST hiking The Narrows. It is simply better for everyone if you try for another day. When the river is low and the chance of rain is below 50%, we feel most hikers can learn about flash flooding risk and manage those risks in a reasonable way. If you plan your hike on such a day, we will help you plan for a safe and prepared experience during our outfitting session.

 

2. Watch for High Ground While Hiking!
While hiking in the Narrows, maintain a keen awareness of the last area of high ground you passed. When you have a sense of how far behind your last safety spot is, you will feel much more confident deciding whether to go forward or backward in the event of flooding signs.
3. Seek High Ground When Flood Signs Occur

While hiking, if any of the following occur, immediately seek high ground 6 feet or more above the river level.

A) Heavy rain begins to fall

B) The river becomes murky and colored from soil run-off upstream

C) The river's flow increases in volume and speed

4. WAIT Until Flood Waters Recede

if you have any doubts about the safety of the river, please: wait, wait, wait. You cannot out-run, out-swim, or out-hike flood waters. Stay put until you are sure it is safe to exit the canyon.

If you are unable to exit The Narrows safely, you are temporarily stranded. Remember, if you cannot get out, help cannot get in. Bring provisions in case of a forced and unexpected overnight stay. You may need to wait 24 hours or more to safely exit after a flooding event.