8.1. SR 06-17-2019 - Laserfiche WebLink

Home Browse Search

Issue Analysis Sedimentation Lake Orono is a reservoir at the downstream end of the Elk River, and receives approximately 3 to 7 million pounds of sediment' from the Elk River watershed each year, resulting in a net deposition of approximately 8000 cubic yards of sediment annually3. This reduces the overall depth of the lake and reduces navigation, aquatic recreational opportunities, and user safety in the lake, and covers vegetation, rocks and gravel, and other underwater structures important for vegetation and wildlife. In 1998, approximately 120,000 cubic yards of sediment was removed from Lake Orono. However, given the lake's position at the bottom of a large, heavily agricultural watershed, sedimentation will continue to be a problem for the lake and dredging to remove excess sediment and restore lake depth will need to be conducted periodically. The study that formed the basis for the sedimentation removal project in 1998 predicted that sediment removal would need to be undertaken again in approximately 20 years, and that prediction has proven accurate. Over the past two decades, mean lake depth has been reduced from 5 feet to approximately 3.1 feet. To restore lake depth to its state after the last dredging activity, the LID proposes to remove approximately 130,000 cubic yards of material. The proposal also states that the dredged sediments will be reused, and it proposes to create a deeper water (up to 10 feet in depth) area near the lake inlet to trap and store sediment. Water levels in the lake may be temporarily lowered by opening the dam to facilitate sediment removal. Sedimentation studies by Wenck Associates suggest that the rate of sedimentation has increased slightly over the past twenty years. In 2011, sedimentation rates were calculated to be approximately 6400 to 6900 cubic yards of sediment, while accumulation rates since 2011 seem to be approximately 8000 cubic yards. This may be the consequence of more accurate and more comprehensive data collection since 2011, or it could reflect an actual increase in sediment accumulation in Lake Orono. Water quality and water flow modeling by the Sherburne County Soil and Water Conservation District suggests that total suspended solids (TSS) in the watershed increased dramatically between 2009 and 2016, due to increased precipitation; this could have been the source of the apparent increase in sedimentation. MPCA is planning to conduct an intensive water quality monitoring program in the Mississippi River St. Cloud watershed, and the information gathered in this program may better address this question. Sedimentation is a known, expected, and predictable ongoing problem for reservoirs such as Lake Orono. The only choices are: 1) let the lake fill in, 2) modify the dam to reduce head and allow more sediment to be transported downstream, 3) stop the sediment upstream of Lake Orono, or 4) periodically remove accumulated sediment. The LID has determined that the periodic removal of sediment is the most practical solution for Lake Orono at this time. The lake management plan states that the removed sediment will be reused. The removed sediment will likely contain curly pond leaf (CLP) turions, and therefore should be used only in upland areas so the turions cannot be introduced into a new water body. The activities proposed to address this lake problem are heavily regulated and require several permits: • Any sediment removal below the OHWL requires a DNR Public Waters (PW) permit, Minnesota Pollution Control Agency (MPCA) sediment disposal permits, and other applicable permits. • Lake drawdowns require consent of at least 75% of affected landowners, a permit amendment, and are regulated under Minnesota Statutes § 103G.408. 3. 2010 Wenck Associates Report on Lake Orono Sedimentation Study. DNR Advisory Report on Lake Orono Lake Improvement District Page 3