Lake Union

Lake Union was formed by the Vashon glacier about 12,000 years ago. The maximum depth of Lake Union is much more shallow (15 meters, 50 feet) than either Lake Sammamish (32 meters, 105 feet) or Lake Washington (65.2 meters, 214 feet). Originally the lake was separated from Lake Washington by the land extending between Portage Bay and Union Bay, and outflow from the lake was through Salmon Bay, which was then a marine inlet.

In 1914, the hydrology of the lake was significantly altered by the construction of the Fremont and Montlake cuts and the Hiram M. Chittenden Locks. These modifications increased inflow to Lake Union by diverting the Cedar and Sammamish rivers and the outflow from Lake Washington through Lake Union via the Montlake Cut and the Ship Canal.

Lake Union now flushes (complete exchange of water) about once a week at high water flows, although there is a significant amount of short circuiting of flow, where the inflowing water does not completely flush the lake before flowing out of the lake, but flows mostly in the north part of the lake from the Montlake Cut to the Ship Canal. At the same time, the opening of the Ship Canal allows periodic influx of salt water from Puget Sound. The actual balance between the salt water intrusion and the flushing rate at a given time varies with the amount of their respective volumes. During the rainy season and spring thaw, runoff from the Cascade foothills is high and the lake is flushed. As the flow drops off in the summer and boat passage through the lock increases, the intrusion of saltwater through the locks increases.

The sediments of Lake Union are very soft, relatively deep, and contain a large amount of organic material. Milfoil and other macrophytes, washed into the lake from the rest of the watershed, contributes a large amount of the organic material. As microorganisms in the sediment break down this material they consume much of the oxygen in the lower part of the lake. By the end of the summer, concentrations of dissolved oxygen in the hypolimnion of Lake Union are near zero.

The intrusion of salt water forms a saline wedge that flows up the bottom of the ship Canal and the lake. Because salt water is more dense than freshwater, the salt water flows along the bottom and is not easily mixed with the overlying, less dense freshwater. The result is less mixing and a much stronger and longer lasting stratification of the lake. The saline bottom water becomes devoid of oxygen early in the summer as the oxygen is used by bacteria consuming the organic sediment. The lake and canal system are the only migration route for the salmonids in the Lake Washington, Cedar River, and Lake Sammamish drainages. The anoxic conditions (lack of oxygen) in the hypolimnion and warm, stressful surface water temperatures limit the amount of habitat available to these fish. The irregular influx of saltwater poses an adaptation problem for the benthic organisms that form the basis of the lake's food web. Concern for these effects and the potential for permanent damage to Lake Washington, should a large flow of salt water move that far, prompted the Army Corps of Engineers in 1964 to build a salt water siphon at the locks to return some of the salt water to Puget Sound. Beginning in the early 1980's the Corp of Engineers made additional modifications in lock operation to help reduce the saline intrusion and conserve Lake Washington water used to flush saline water from the locks.

Physical characteristics
Lake Union and its Drainage Basin

Drainage Area	600 miles2 (384,000 acres)	1,554 km2
Lake area	580 acres	2.3 km2
Lake volume	20,000 acre-ft	2.5x107 m3
Mean depth	34 feet	10 m
Maximum depth	50 feet	15 m
Flushing rate	52 per year
Depth of epilimnion	40 feet
Epilimnion:Hypolimnion ratio	32.3
Percent hypolimnion	3%
Main inflow	Lake Washington Ship Canal
Main outlet	Hiram Chittenden Locks
Trophic state	mesotrophic
Minimum dissolved oxygen in hypolimnion	0 mg/L

Lake Union is unique among the three local major lakes in the character of its watershed. It is the most heavily urbanized of the three, draining residential, commercial and industrial neighborhoods. Its shores are completely lined by marinas, house boat moorage, commercial docks and dry-docks and industries. In the past, the lake has received sanitary discharges from houseboats and ship and industrial discharge from businesses along the shore, as well as fuel spills and discharges from ships and onshore facilities. While pollution inputs from many of these sources have decreased, not all of these discharges have stopped, and combined sewer overflows (CSOs) still overflow into the lake and are generally agreed to be the worst source of pollutants and pathogens to the lake.

To address the pollution from the CSOs, a Combined Sewer Overflow Control Program was begun in the late 1980's. This lead to the separation of storm water from the University Regulator drainage basin with a new storm water discharge to Lake Union in 1994. A study has been underway to assess and compare impacts of the combined sewer, of the new separated storm water discharge, and better understand the overall ecology of Lake Union. Data collected prior to the sewer separation will be compared to data currently being collected after the separation to asses the toxicity potential of the storm water discharge, and the chemical and biological impacts of the discharge to the water column, sediments, fish and crayfish of Lake Union. Evaluation of the efficacy of this work is currently underway to determine if external phosphorus loading, heavy metals, and priority organic pollutants in the water column and sediments are controlled by this design.