Field Notes

June 10, 2020

How many people does it take to raise lake water levels by one inch? And other water level questions

By Jed Harvey in collaboration with Abby Phinney, Lilly Center Staff

Imagine this: It’s late February, 1965, and you live near Lake Wawasee or Syracuse. It isn’t cold enough for the lakes to be frozen; in fact, the water in Wawasee and Syracuse reaches far higher than normal. Water is gushing in great volume out of the newly built dam in Syracuse and into Turkey Creek, but that doesn’t keep the lakes from cresting over seven inches above their normal levels. You decide to check on a friend at a nearby lake, so you pull out your phone book, wait for the party line to free up and dial your friend’s number into the rotary phone. Just a few miles away at Dewart Lake, your friend picks up the call and agrees that Dewart does, in fact, have an issue with water, but not in the same way. Despite Wawasee having far too much water, Dewart has far too little, leaving your friend’s pier high and dry!

As strange as it sounds, this disturbance in the lake levels did actually happen. According to an article published on Monday, Feb. 22, 1965 in the South Bend Tribune, Lake Wawasee and Syracuse were seven inches above normal levels while nearby Dewart was far below where it normally sits. The article gives some clues as to why this happened, but the specific details of why Wawasee was so high and why Dewart was so low have likely been lost to time. Luckily, we have just enough information to use them as great examples in explaining how and why lake levels fluctuate.

Why do lakes fluctuate?

Just like businesses and bank accounts, lake water levels naturally change due to budgeting. A lake’s water budget is just what it sounds like: all of the water going in as well as all of the water going out. “Inflows” include most or all of the ways water enters a lake, such as inflowing streams, rain and underground springs. “Outflows” include outflowing streams, percolation through the ground and evaporation into the air. Changes in the ratio of inflow to outflow effect the general level of lakes. If you close a dam, thereby reducing the outflow of the lake, the levels will rise. If you divert a stream away from a lake, thereby reducing the inflow, the lake levels will fall.

Let’s look at Dewart and Wawasee back in 1965, starting with Wawasee and Syracuse lakes. While we can’t easily find the exact factors that played into why the pair were so high, we have a few crucial scraps of information that will be helpful as we piece together the mystery of the extra-and-also-not-enough water. It was late February, and from that we can assume that there was likely melting snow in the Wawasee watershed as well as possible rain. Both of those can have a delayed effect on lake levels, but if you read our article about flooding, you will know that rain and melting snow can contribute quite a lot of water to a system, sometimes even creating dangerously high water levels. From the date, we can also gather that the lake was cool, which means that not a lot of water was lost due to evaporation. Together, that extra input and slight loss of output probably factored in on making Wawasee and Syracuse raise to flood-level.

Dewart, on the other hand, has more information to draw from. The summer before was dry, so the lake was likely already starting at a deficit. Additionally, there was a leak in the lake, an exposed gravel vein that the water was flowing into. As more water flowed out of Dewart, it likely got so low that not even a good snowmelt could correct the water level.

How does a raised water level impact a lake?

Compared to normal lake levels, high levels can have surprising effects on the lake as a whole. While it is easy to see some of the major effects like flooding and filling yards and basements with water (see the flooding article for more on that), there are things happening beneath the surface that are surprisingly impactful. During high water levels, bank and shoreline erosion increases, which does a disservice to the structure and stability of the lake’s edges. It also washes soil and sediment (along with any fertilizer, pesticides and herbicides that were applied to the area) into the lake, and can greatly increase the nutrient level of the water.

How much does rain effect lake levels?

Rain effects different lakes in different ways. All lakes are unique, so there’s no standard equation or formula to say how much rain it takes to maintain or raise a lake’s level. In theory, if a lake had perfectly vertical sides and a smooth bottom and had no inflows or outflows (like a pool) one inch of rain would raise the lake’s level by one inch. But lakes are far more interesting than in-ground or above-ground pools: Sloped sides, different types of shoreline, large or small watersheds, soil types, and upstream lakes are just a few of the items on the list of factors that will change how much or how quickly rain enters the lakes.

Will jumping off a boat change the level of the lake?

Short answer: No, not really. Jumping off boats doesn’t affect water levels. The most important part of the phrase is “off a boat” because of a thing we call buoyancy. While you are on a boat floating in the water, your weight pushes the boat down into the water just a bit. Buoyancy means that as the boat floats, it’s pushing on the water and is able to move just as much water as the boat weighs. If you put a bowl in the water, it will sink a little, but if you put a bowl of rocks in the water, it will sink a lot more (maybe all the way) because the bowl of rocks weighs much more and can push more water out of the way. In the same way, a boat floats, but when a person gets on, it floats a little lower, proportional to how much the person weighs. When the person jumps off into the water, the boat raises up a bit, but a similar amount of water is moved when the person enters the water, so the water level change is basically zero.

Now, there would be a slight change if one did an epic cannonball. Assuming that you could do the perfect cannonball, making a sick wave that soaks the lakeshores, there would be a change. It would take one crazy (absolutely impossible) wave to make a difference that could be measured, though.

…okay, what about jumping off the shore?

Yes, there would be a small change in lake water levels. People aren’t really that big, at least not compared to a lake, so jumping off of the shore would have an affect on the water level but not one that you would be able to measure, similar to doing a cannonball. And if you did a cannonball off the shore and made a monster wave, it wouldn’t have a substantial impact on the water level.

Oh, well then, how many people would it take to make such a change?

This question was brought up by a loved one as we discussed this topic over dinner (as I am wont to do); they asked, “How many people would have to jump into a lake together to raise the level, say, an inch or so?” At first I thought “Hmm that’s silly, I would never spend time doing that math…” but then, an hour and a half later, I have the numbers (and have had them double and triple-checked!)

Let’s use Winona Lake as our basis. I won’t bore you with the math, but assuming that all the participants have the same mass (about 175 lbs), fit together like perfect puzzle pieces, breathe water (which is an issue we’ll have to work out while planning the hypothetical study for this), and that the lake has vertical sides (it makes the math easier), the approximate number of people needed to raise the surface of Winona Lake by merely one inch is…

Seven-hundred and fifteen thousand people.

715,000.

That’s the entire populations of Kosciusko, Allen, Elkhart, Marshal and Noble counties combined, all jumping into Winona Lake at once. That many humans in Winona Lake would only raise the surface level of the lake by one single inch. And if that isn’t bad enough, the numbers for Lake Wawasee are truly staggering. Based on the same assumptions, it would take 25,000,000 people (the entire city of Shanghai, or two entire Los Angeles’s) to do the same to Lake Wawasee!

Lakes are complicated. There’s a lot that goes into them, there is a lot that comes out of them, and there’s a lot that goes on that we can’t see. In fact, that’s why we do what we do at the Lilly Center. We regularly collect data on 14 lakes and 12 streams to monitor water levels and much more. We are on-mission to understand and help protect local waterways by giving local government, lake associations and businesses, and you all the information needed to make the best decisions for our community and lakes.

Do you have questions like the ones we’ve answered here? Send any and all to lakes@grace.edu and we will do our best to answer them!

Here are some helpful resources for further reading: