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Identifying Floodwater

River of the month #79

May 2005

By Kris Wolpert and Pat Reilly

FLOOD WATCH! This alert strikes fear into the hearts of riverside dwellers. But to many boaters it means opportunity. A lot of us get filled with anticipation when we hear the forecast, ‘Rain, heavy at times; small stream and urban flooding.’ Exploratory and whitewater paddlers know that the number of streams available to run increases exponentially with each half inch of rain.

Recreational boaters are taught that floodwaters greatly increase risk along with opportunity. Flooded water really does change a river. Drastically! ‘Serenity Brook’ can turn into ‘Wildass Torrent’ in a hurry. So when the deluge hits and all the little creeks are finally up, how do we define that thin line between, ‘this is the chance of a lifetime!’ and ‘I must be out of my mind!’

Water in the trees? This may or may not be indicative of flood. Bigger rivers have shoreline species like birch and willow that can be surrounded by water long before the river is really in flood. Some tiny creeks like York County’s Beaver Creek (ROM for March, 1999) must have water in the trees at the put-in or there won’t be enough water to get down through the boulders. Flood or not, it stands to reason that water in trees is time to exercise extreme caution and good judgment.

Water out of its banks is a better indication of flood. This is actually the definition of a flooded river. Streams usually have well defined banks, but keep in mind that rivers only need to flow from their banks in a few spots to begin to exhibit flood characteristics. With the Susquehanna 2 feet above flood level, it is still within its banks at all but 2 spots in Harrisburg - Shipoke and West Fairview.

Noting how much debris is in the water doesn’t tell whether or not a river is in flood, but it will give us an important piece of information. Besides determining if there is simple too much debris to paddle safely, you can tell whether or not the water has crested. Floating trash is prevalent on high rivers right up until the level tops out. Then it seems to greatly diminish. Boaters that like to play in the Susquehanna’s big waves at Falmouth during high flows generally wait until the water crests so that they aren’t surfing their favorite wave in competition with floating trees, out houses, drowned cattle, etc.

Don’t forget that you can, of course, use the gauges! The USGS realtime river gauge web site, accessible directly or through links from dozens of paddling sites, gives the definitive answer. That is if the river in question has a flood stage. Though many smaller streams on the website don’t have established flood levels they can still be monitored for flood information by checking how high the flow is above the norm for the date. But remember to take the time of year into account. If it’s 10 times the norm in winter or 20 times the norm in summer, it is probably near flood. (We’re talking cfs, not gauge height) You’ll have to look at the period of record too. If the gauge has only been in operation for a few years, the norm may not reflect a true average.

The most important indicator of flood level is what the water is doing. 𠆌lass V in the doldrums!’ is the old adage - a sure indicator of flood. There may be rapids pop up where there was normally flat water while sections of rapids may wash out to smooth flat waves. Weird! Floodwater also tends to go ballistic! That is, it tries to take a bullet’s path, straightening out bends and attempting to re-shape its bed, slamming up against banks and anything it its path. Watch out!

Floodwater will generate extremely fast current and on smaller rivers, if the water has any gradient at all, a series of standing waves will run down the middle of the creek. When we find ourselves on smaller creeks in flood, we’re constantly back paddling to scrub off speed while flying down through rolling waves that sometimes run the entire length of the flooded water. The change seems to occur suddenly when a stream floods. There is something happening here that takes a hydrologist to explain, but the difference is dramatic:

Near bank full – 4 mph current, fast but manageable, ‘yee-haa’, time to hammer out record speed.

Flooded – 8 mph current, constant waves, eddy lines that’ll spin you on dime, ‘whoa, back off’, what’s around the next corner?

In these conditions the normally safe haven of the shoreline often disappears, replaced by a sieve of flooded trees! With its high velocity flow, floodwater has a tendency to scatter swimmers, their boats and gear for miles. So even if you survive an out-of-boat experience, you may never see you boat or paddle again!

Bigger water with little gradient, like the Susquehanna at Harrisburg or Conodoguinet, may not generate these long wave trains and can look flat and inviting at flood but they have their share of dangers. When paddling upstream on the Susquehanna at flood level we find that riverbank eddies are sometimes huge and look calm but can be dynamic. Big, maybe 20-foot diameter, sections of water will shift eerily underneath you with little noticeable surface disturbance! You quickly find yourself entangled in shoreline tree branches when you were 10 feet away a moment ago. Again - weird!

On these bigger waters everything is greatly increased. The rocks in places like the Dauphin Narrows may be submerged but the waves will be monstrous. And if there is a rock or ledge big enough to cause a pourover, it will form a hole of planetary proportions! Big waves may be generated at normally placid spots, like between bridge piers. There seems to be no logic to it. Rapids tend to migrate downstream. At Falmouth, the water is super fast with just big flat smooth waves until halfway down through the rock ledges, where it explodes. Utter chaos then continues for a half mile below the normal end of the rapids. At least Harrisburg’s dangerous Dock Street Dam disappears completely with not so much as a dimple on the surface.

Keep in mind that floods cause sewage treatment plants and industrial sediment ponds to overflow adding some truly ugly unseen components to the water. Low clearance bridges and culverts can create suck holes that are certain death traps. Then there’s big hungry reaction waves bouncing off of rocks, cliffs and bridge abutments, compliments of the ballistic flow. Don’t forget wire fences and cables or power lines normally above your head that are now at neck level. Not to mention eddy fences downstream from bridge piers that can re-circulate you like a keeper hole. Remember that all of these hazards will be coming at you at more than double the normal current speed.

Whew! Sound ugly? It can be, so why paddle flooded rivers? Opportunity. Some, like Beaver Creek, need to be at or near flood just to navigate them. Again, it gets back to what is becoming common advice in this column – learn to honestly evaluate your equipment and skills. With flood levels, you’ll need to add a healthy respect for much faster moving water and to be ready for anything, literally! Flood levels are not for beginners and it should go without saying that your skills need to be considerably above what is required to navigate the water under ‘normal’ conditions. Even then, it is a bit of a crapshoot.

Fishing Creek (April 2000), Horse Valley Run (Sept 2000), and West Branch Mahangtango west (April 2004) are additional ROM columns that talk about floodwater.

Pat Reilly

Copyright © 2005 Pat Reilly.  All rights reserved.

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