Lake Level “Bounce” Explained
UNDERSTANDING LAKE LEVEL “BOUNCE” ON BIG PINE LAKE
Anyone who has spent much time on or around Big Pine Lake in Crow Wing County knows that the lake level varies widely and fluctuates often. “Bounce” is the term used to describe the difference between high and low lake elevations in a given period of time. During the past seven years, measured bounce on Big Pine Lake has been as little as 24” during an entire summer season to as much as 48” in just two weeks. By comparison, that is at least 4 to 8 times greater than the bounce on any of the 14 lakes in the Whitefish Chain just a few miles away.
This magnitude of bounce presents real challenges to lake property owners in several ways. Historically, the sustained periods of low water elevations have caused some property owners’ docks, lifts and boats to be sitting unusable on dry lakebed. Other property owners around the lake may have their boats “stuck” on their lifts with insufficient water depth to launch them. During the relatively brief periods of extremely high water elevations, some property owners experience flooding of their lawn and landscaping nearest the shoreline. Boats and dock sections can be dislodged from their moorings and some boats can become “stuck” under lift canopies. And, of course, the frequent movement of the lake elevation between high and low levels often causes significant shoreline erosion.
While most lake property owners have learned to cope with the consequences of both high and low water elevations, such radical conditions are not desirable and do little to enhance property values.
So, why is the “bounce” on Big Pine Lake so large? The simple answer is that the extreme and frequently changing elevations on Big Pine Lake are the direct result of local rainfall and the operations of the US Army Corps of Engineers’ Pine River Dam in Crosslake. By surface area, the Whitefish Chain is 36 times greater than Big Pine Lake so it acts like a huge amplifier of the effects of any rain event. An inch of water on the Whitefish Chain is equivalent to one yard (36 inches) of water on Big Pine Lake!
That’s why the Big Pine Lake Association (BPLA) Board members have been working with the US Army Corps of Engineers and other local agencies to better understand and manage bounce on Big Pine Lake since 2013. The remainder of this article explains the two major contributors to bounce, as well as, what has already been done and what is planned to better manage bounce in the future.
First, it is important to recognize and accept some elements of the radical bounce problem that are “givens” and cannot be changed or controlled. Big Pine Lake is located roughly 2.5 miles (as the crow flies) downstream from the US Army Corps of Engineers’ Pine River Dam in Crosslake, MN. That large dam controls the elevation of the entire Whitefish Chain of 14 interconnected lakes. The operational staff opens and closes any of 13 large gates to release water at various outflow rates to maintain the “pool elevation” of the Whitefish Chain within a “Summer Band” of only 6”. The first (and primary) contributor to bounce on Big Pine Lake is the outflow of that large dam which becomes the primary inflow to Big Pine Lake via the Pine River. The water levels here on Big Pine Lake correlate directly and strongly with the published outflow rates from the Pine River Dam. This has been proven mathematically and shown graphically using many hundreds of daily elevation measurements and daily outflow rates over the past five years (see example charts below).
Notice how closely the elevation of Big Pine Lake (yellow solid line) follows the profile of the outflow rates of the Pine River Dam (vertical blue bars). This example shows the actual data for July, 2016 when the maximum monthly bounce of over 2.5’ occurred in just one week as the result of six cumulative inches of rain during that period (see chart below).
That rain event caused the pool elevation of the Whitefish Chain to rise above the top of the Summer Band so the Pine River Dam operations staff methodically opened the gates, increasing the outflow over 42 times from 33 cfs (cubic feet per second) to 1,400 cfs (see chart below). Even at that high rate of outflow it took a full week to bring the pool elevation of the Whitefish Chain back to the top of the Summer Band. Meanwhile, the elevation on Big Pine Lake rose high enough to submerge many docks, strand boats on lifts and flood some lawns and landscaping adjacent to the lakeshore, especially on the south side.
The second major contributor to bounce on Big Pine Lake is the design and condition of the water control structure on the Pine River that currently forms the lake. In 1970 a primitive, loose rock dam was built across the Pine River in an attempt to assure a minimum lake level during dry periods. Still, for the past many years, some property owners on Big Pine Lake have experienced long periods of such low lake levels that their docks are sitting above dry lakebed, completely unusable. Since the operations staff of the large Pine River Dam in Crosslake is required by law to release a minimum outflow of 33 cfs, Big Pine Lake has a guaranteed inflow and should never “dry up”. However, if the rock dam has porosity leaks, has been breached by high outflows and/or floating bogs, or if the top of the dam and/or spillway have been severely eroded, then the level of Big Pine Lake can become excessively low, even with a guaranteed minimum inflow.
While the high lake levels are primarily dependent on the amount of rain and Pine River Dam outflow rates, the low lake levels are dependent on dam design and maintenance so the Big Pine Lake Association Board focused on those issues first. By working closely with the Crow Wing County Engineering Department (Crow Wing County owns the rock dam) to repair breaches and erosion and restore the dam elevations to their original permitted specifications, the BPLA Board has now virtually eliminated excessively low water conditions (see July 2018 example below) which has also reduced the bounce by 12 to 16”. That means that all lake property owners anywhere around the lake will have water under their docks and can position their docks and boat lifts each spring knowing that, failing a catastrophic breach in the dam, lake levels will not drop below a minimum desired elevation during the summer season.
Since 2013, the BPLA Board has also been working closely with Crow Wing County on a much more durable and reliable replacement for the old rock dam which was built in 1970. Progress has been slow but steady in designing a completely new arched-riffle fish passage structure that will maintain usable lake levels in Big Pine Lake while withstanding high outflows from the Pine River Dam and still allow fish to move freely up and down the Pine River at all times. Although this project is supported by lake property owners, Crow Wing County, Crow Wing Soil and Water Conservation District and the City of Crosslake, it has proven to be more costly than originally estimated. Therefore, initial grant funds were insufficient and a new application has been submitted for a much larger grant to cover the actual construction costs. Assuming that grant is awarded in January of 2019, construction of the new fish passage structure should occur sometime within a year. Extensive engineering design work has been completed, including detailed modeling of Big Pine Lake and the Pine River Basin, to ensure that the bounce from high outflows will be no more than produced by the old rock dam. Thus, the future bounce experienced on Big Pine Lake, although still significant, will be of lower magnitude and more predictable. Best of all, the replacement structure will be much more reliable and require much less maintenance and expense.
In summary, Big Pine Lake, by the very nature of its location downstream of the Whitefish Chain and Pine River Dam in Crosslake, has always been and will always be exposed to relatively large bounce and frequent lake level changes. However, a great deal has been done in recent years to understand and communicate the specific contributors and relationships that cause these conditions. Also, much has already been done and more is planned to be done to control and reduce bounce on Big Pine Lake. All current and prospective property owners on Big Pine Lake should understand the lake’s unique characteristics, as well as, the primary contributors, and what is being done to manage them.
Credit: Above article courtesy of Dennis Bengtson, Immediate Past BPLA President