The Dock Street Dam

River of the month #92

author: Pat Reilly
date: November 2006

We hear so much misinformation about the Dock Street Dam in the Susquehanna River at Harrisburg that I felt compelled to try and clear some of it up. To begin this discussion, let’s emphatically state that this dam is very dangerous to boaters and swimmers. It is a classic ‘drowning machine’ dam, so named because they form a hydraulic that at certain levels is inescapable. When the USGS Susquehanna River gauge at Harrisburg, located on City Island, is between 3.7 and 6.5 feet, this dam is lethal - it can trap, hold and re-circulate swimmers, possibly drowning them! The water above the dam is generally too deep to stand in and the current too strong to swim against. And with the river 7 tenths of a mile wide at the dam it can be a long way to the safety of the shoreline for paddlers out of or not in control of their boats. It is helpful to know that the water for more than a mile above the dam is flat and the current will not exceed 2.5 mph at levels below 6.5 feet. But beware the wind! It often sweeps strong downriver and can grab hold of your boat (especially big open canoes) and push you over the dam to your death! In other words, stay the hell away from the dam, unless you are 100% positive that you can turn and paddle away from it.

Below the dam can be equally perilous. Curious boaters can get too close, cross the boil-line and be sucked into the hydraulic. This scenario has killed quite a few fishermen attempting to cast into the productive turbulent waters of the dam. As paddlers we know that the boil-line is the spot that divides downstream flow from water that is kicking back upstream toward the dam - the re-circulating water. With the gauge at around 6 feet, the Dock Street Dam’s boil-line is about 25 feet from the dam! Way further than one would ever suspect. Keep well away!

If you’re unsure just what a re-circulating hydraulic looks like or how one works, you can get a duck’s eye view from the riverside steps at Harrisburg’s Shipoke neighborhood. The walkway above the steps is part of the Harrisburg Greenbelt and there is parking under the South Bridge (I83). The dam is just north of the bridge. Be sure to note the gauge level when you take a look. If the hydraulic is really working you may see trees, 55-gallon drums, one of my lost boats or just about anything that floats re-circulating below the dam, bouncing around, bobbing up and down, sometimes for days.

Growing up in Wormleysburg, I learned a healthy fear of the Dock Street Dam at an early age. When I was very young a neighbor drowned at the dam. The town talked about this tragedy for years and it was pointed out to me nearly every time the river was mentioned.

We should all know that it is illegal to approach to within 200 feet above or 100 below this or any other dam over 200 feet wide. Even the runable spillway on river left has been deemed off limits by the current Pennsylvania Fish and Boat Commission conservation officer in charge of the Harrisburg pool. Now, that being said, and having pointed out the extreme danger of the dam, let’s consider the rest of this column to be simply narrative of one idiot’s experiences. This is not meant to be advice and the CCGH does not condone the following activities.

Most paddlers are led to believe that the dam is dangerous at any level. Nope! The levels at which the hydraulic below the dam might re-circulate a swimmer form a relatively narrow window – from about 3.7 to 6.5 feet. Trouble is, these levels are very common and the river is within this narrow window for most of a normal year. Let’s look at what happens above and below these levels.

I’ve always considered 3.2 feet to be the ‘bottoming out’ level for the river. This is sort of a base low. The river will get lower, for sure, but at some point nearly every summer and into September the river will hit this level. At 3.2 feet there is maybe 1 inch of water flowing over the dam. This is simply not enough water to generate a hydraulic, just a river shower. The top of the dam is wide and consists of corroding concrete providing good traction and making it easy to walk across the dam. As kids we would swim down to the dam at low levels, get out and walk across the top of it. As an adult I would fish from the dam and remember my dog following me out to the middle of the river walking on it. However, I would strongly advise against swimming to the dam now. There are at least 2 holes in the dam that create suck-holes that you could conceivably be stuffed into as a human plug.

The dam is formed from a slab of concrete slopping downstream at a shallow angle. The slab is held up by walls spaced about 12 feet apart. This creates air space between the walls forming little rooms. For as far back as I can remember, at low levels us kids would duck under the curtain of water falling from the dam into these secret rooms. We thought we were pretty cool sitting there with the dam slopping up behind us, the walls on either side and a shower of river water splashing down in front of us. It was fun to stick a hand or your head out from under the water curtain so it looked disembodied to observers on the outside – just a laughing head protruding from a wall of water.

With the head of this dam no more than 4 feet (as paddlers we know that the head is the difference in water levels above and below a dam) you must duck to fit into the rooms. But I found that at the right low level I can paddle my creek boat (an Eskimo Cerro K-1) under the wall of water and it will just fit with me in it. The rascal in me thought of using this disappearing act to play a game of hide and seek with the Fish and Boat Commission officers when they chase me for playing at the dam. But that would no doubt be inviting more trouble than it’s worth.

At low levels, below the dam is a fantastic place to snorkel during those rare cases when the river gets clear enough to dive, usually in September after a cold snap knocks back the suspended algae. I like to go over to the river left side, run the spillway, park my paddle craft on exposed rocks under the I83 bridge and snorkel up to the dam. The bubbly froth seen from underwater in a bright sunlight is a sparkling treat for the eye. You can swim under the curtain of water and in and out of the little rooms while checking out how seriously undermined the 100+ year old structure is in spots. I’m no engineer, but I would not be at all surprised if sections of the dam were to collapse tomorrow. It’s that bad in places!

But the best thing about snorkeling here is the fish! Schools of every kind of fish found in the river congregate in large numbers below the dam to take advantage of increased oxygen and good cover. Among others, I’ve seen schools of walleye and crappie, 2 species rarely found in most parts of the river, and a big albino catfish!

So much for goofing off at the dam, this column is supposed to be about paddling. The dam does not have to be the obstacle to river travel that authorities seem to make it out to be. With modern emphasis on litigation and protecting us from our own stupidity, it’s no wonder that the PFBC doesn’t want you anywhere near the dam and guidebooks and maps steer paddlers well away from it. An example is the map from the Lower Susquehanna River Trail. To funnel paddlers to the Harrisburg shore, where they can portage along the concrete steps that form the city’s riverbank, the map lists the entire river right side of City Island, from the island to the west shore, as off limits. The reasoning here is to prevent out of control or capsized boaters from drifting over the dam. While this is a real concern, paddlers who are in control need not get to river left when they are still a mile from the dam! Indeed, if I where a ‘responsible’ paddler and followed the map, I’d never launch a boat from my dock, as the riverbank in front of my house falls within the trail map’s off-limits area! For that matter so does the City Island boat ramp, where most paddlers launch!

There are 3 ways a paddler can get over the dam in his/her boat. Actually 4 ways, you can scrape right over the top if the level is low, but you must know the depth on the other side, which varies greatly and is as low as a few inches. The cheap thrill from this option wears off quickly, believe me. You want to run waterfalls? Go to West Virginia.

River left is the spillway, about 20 feet wide, extending out from the concrete steps. In spite of the PFBC’s current standing on running it, it was probably designed for boat passage. It’s feasible to run at any level. The lower the water the gnarlier the drop, but it’s really never more than a good class 2. At low water it is very pushy and powerful but straight forward with no hydraulics or rocks to avoid. If the big diagonal breaking wave doesn’t flip you, you’ll probably be okay. Two words of caution – first, you obviously want to stay river left and avoid going over the dam proper if the hydro is working! Second, there is so much fishing activity here that the bottom of this rapid is littered with lost tackle and fish hooks. If you’re not sure of staying upright, take the easy carry on the steps (or line your boat through). At higher levels, the spillway smoothes out to small waves and fast water.

Surfing here is a possibility. The diagonal wave is tough to stay on, but at real low levels, there’s a bit of a play wave/hole formed. It’s hard to get on, if you flip the water depth is low, and it tends to surf you into the concrete on the shore. But hey, all surfing spots have their problems. We usually play here late at night when the fishermen (and the PFBC) are gone.

A riverbed raised by ledge rock causes the extreme right side of the dam to drop no more than a foot, so passage is possible over there too. At low levels you will clunk against the dam and if you manage to slide over, you’ll land on wet rocks. At about 4.5 feet it’s a go but you may bang and scrape. Over 5 feet it’s clean, just slip right over and bop on down through the riffles to the bridge. Over 6 feet it’s smooth enough to paddle back up! The important thing here is to stay tight right, so tight you can touch the shore with your paddle. Go out any further, and you stand a chance of banging hard, maybe pinning or even getting surfed into the hydraulic. This move may sound hairy on paper, but it’s really not, if you’re in control and stay tight right. I use this route dozens of times a year in all types of paddlecraft. If you must portage, it’s short but ugly with tangled brush and a steep bank of railroad ballast stone.

Now for the center spillway! Many river folks, even some long time boaters, are not even aware of this narrow passage in the middle of the dam. It is found in line with a downstream island that extends almost up under the I83 bridge. Many that are aware of it say that it is a break in the dam. But it’s not, when water is extremely low, you can see that it was built into the dam. Why, I do not know. If it too was for boat passage, those early 20th century boaters must have had whitewater craft. This drop is big, abrupt and powerful at low levels. At your summer bottom-out level of 3.2 or below, the drop is a smooth tongue of water falling at a steep angle, accelerating quickly and exploding in a big pile of foam. It’s a real hoot to run! Too bad it’s over in about a second. I enjoy taking unsuspecting guests out in the bow of my K-2 and running them through the drop. The foam pile slams them hard in the chest and assures a good soaking. Again, it is straight forward and the foam pile would never hold a boat, so the only real danger is getting beat up in the rocky run-out if you flip. Of course, it would be dangerous to approach with the river at a level where the dam’s hydro is working - miss the spillway and die. So don’t bother, the center spillway smoothes out to smaller breaking waves as the water level rises.

I’ve played in the foam pile a few times at low levels. It is surfable, but being so shallow and so powerful, a flip could rip your head off, figuratively if not literally. There’s an old story of a CCGH paddler that flipped his K-1 while surfing the center spillway at low level and promptly came up missing a blade from his paddle. Then there’s a more recent story (last spring) of another member who garnered a fat fine for practicing rodeo moves at a level slightly above 4 feet. This level provides good play but the hydro is working on either side of the spillway so you’d better know what you’re doing! Anyway, our rodeo star was the only one targeted even though there were fishermen clearly within the 100-foot limit at the same time. The officers never launched a boat but waited for him at his car parked under the bridge.

Alright, we’ve talked about low levels, what happens when the gauge at Harrisburg exceeds 6.5 feet? Bye bye hydraulic and hello waves – first, a series of breaking waves that may be playable with very short boats. Then as the level rises the waves smooth and flatten quickly until they disappear all together somewhere around 14 feet. That’s right! no more dam, not even a dimple on the water. On river right at a narrow window somewhere between 6.2 and 6.6 feet, Dave Ertel and myself found good play opportunity in some smooth surfing waves, some with gentle foam piles good for 360 spins. Granted, that’s a narrow window and those types of waves only exist on river right where the ledge rock keeps the dam head lower.

I guarantee that at 7 feet or above you can run anywhere over the Dock Street Dam and will not get caught in a hydraulic. An inept boater, or one angling sideways, may get flipped in the waves, but no one is going to re-circulate at those levels. But don’t be a fool, look it over (for that matter look over everything you run). You can’t be sure of the exact level at the exact time you run until examining the data after the fact.

So there you have it, probably more than you ever wanted to know about the Dock Street Dam. Yes, it can kill you, and will kill you if you tangle with its hydraulic. But with a little knowledge, reasonable boat control and the current Harrisburg gauge reading, you don’t always have to run and hide from the ‘killer’ dam.

Pat Reilly

Copyright © 2006 Pat Reilly. All rights reserved.