Scientific Investigations Report 2006–5209
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5209
Upper Klamath Lake is shallow—92 percent of the area of the lake is less than 4 m in depth, 65 percent is less than 3 m, and 20 percent is less than 2 m. The 4 percent of the area of the lake that is greater than 5 m depth is found in a narrow trench along the lake’s western shore (fig. 1). The southernmost extent of this trench is at roughly the latitude of Buck Island; it runs across the entrance to Howard Bay, along the western shoreline and then parallel to Eagle Ridge to the west of Bare Island. At the tip of Eagle Ridge, the trench turns west and runs across the entrance of both Shoalwater and Ball Bays before turning north again and gradually fading away. Maximum depths in this trench approach 15 m. The bathymetry of the lake plays a prominent role in defining the wind-driven circulation and has consequences for water quality as well.
Within the northern one-third of the lake that constitutes the study area, there are less extreme variations in the bathymetry that can influence observations at the continuous monitors. The deepest part of the study area (excluding the trench) is to the south and west, up against Pelican Bay to the west and Shoalwater and Ball Bays to the south. The lake bottom shoals upward from there to the north and east, so much so that site UKL01 could not be maintained for the entire field season as the lake level declined. To provide a quantitative measure of the difference in depth across the study area, the depths at sites UKL10, UKL07, and UKL02 were 3.8, 4.5, and 2.9 m, respectively, on June 1, 2003.
Being a shallow lake, Upper Klamath Lake does not develop a strong thermocline as would be observed in much deeper lakes. Nonetheless, some degree of thermal stability can, and often does, develop in areas of Upper Klamath Lake that are deeper than about 2.5 m. This thermal stability is most resistant to mixing in the trench, but it routinely develops and erodes over the course of a day in areas of the lake only 2.5–3 m deep. During 2002, data from 1 m off the bottom and 1 m from the surface were collected with profiling buoys at sites UKL07 and UKL08 (fig. 2). The buoy at site UKL07 remained there during all 3 years of the study, but in 2003, the second buoy was moved to site UKL13 (fig. 3), and in 2004, it was moved to site UKL16 (fig. 4). The depth at site UKL07 varied by almost a meter between the 3 years, but was comparable to site UKL08 in 2002 and to site UKL13 in 2003. In 2004, the depth at site UKL16 was much greater than that at site UKL07 (table 1).
Data from the profiling buoys show that a maximum daily difference in temperature of 1–3°C between 1 m off the bottom and 1 m from the surface is common and, in fact, this amount of thermal stability develops nearly every day during the summer. These data also show that the daily minimum difference in temperature between 1 m off the bottom and 1 m from the surface is usually zero (depicted by a thin red line in the figures)—that nearly every day the stratification that develops is eroded at some point either by wind mixing or cooling at the surface or both. Wind speed typically picks up in the late morning to early afternoon, reaching a maximum in the evening and then dying down in the early morning hours, with calm conditions persisting through the morning. Thermal stability typically develops and increases throughout the morning and afternoon, and then collapses in the evening or early morning. Occasionally a period of calm winds will result in stratification persisting for several days, particularly at deeper sites such as UKL16, which is located in the trench (fig. 4). This daily pattern of thermal stability provides ideal conditions for the growth of AFA, which is able to take advantage of thermal stability to regulate buoyancy in order to position cells optimally in the water column (Oliver and Ganf, 2000). Thus, the typical pattern of diurnal stratification/destratification in the lake probably has contributed to the ability of AFA to out-compete green algal species in the lake.
The depth of the photic zone varies with the amount of AFA in the water column, but typically is between 1.5 and 2.5 m during a moderate to heavy bloom (fig. 5). Consequently, much of the lake is shallow enough that nearly the entire water column is continuously within the photic zone. The areas this shallow include Howard Bay and the open waters of the lake east of Howard Bay and south to the lake outlet, as well as the northern and easternmost parts of the study area, including south of Agency Straits and the entrance to the Williamson River.
Because observations of water quality, meteorological variables, and currents were made simultaneously in this study, an increased understanding of the wind-driven circulation in the lake has emerged. Two acoustic Doppler current profilers (ADCPs) were placed to the east (ADCP8) and west (ADCP4) of Bare Island in 2003 by investigators at the USGS National Research Program. A basic description of ADCP operation and the type of data they collect is provided by Gartner and Ganju (2002). During prevailing wind conditions (wind from the west to northwest, between about 270 and 300 degrees), currents at the trench site (ADCP4) were consistently aligned toward about 350 degrees, just west of due north, while currents to the east of Bare Island were consistently aligned to between 120 and 150 degrees, to the southeast (fig. 6). There were periods of time lasting from one to several days when the wind reversed and came from the east to southeast. These periods are seen in the ADCP data as times when the currents also reversed direction.
In 2004, an ADCP was put in the midtrench area at ADCP1 from June 16 through August 6, and then it was moved to ADCP4 for the next deployment from August 6 to September 15 (fig. 7). A second ADCP was put in at ADCP2 from June 16 through August 6, and then moved to ADCP3 from August 6 to September 15. The currents at both sites in the trench were aligned with the bathymetry in the trench, flowing north under prevailing (west to northwest) wind conditions, but reversing direction for periods from one to a few days when the wind direction changed. Currents at site ADCP2 were low and highly variable in direction, flowing primarily to the south and southeast, occasionally turning northward briefly during a particularly strong wind reversal. Currents at ADCP3 were consistently northward, aligned with the end of the trench as it turns northward at Ball Bay. There were no strong wind reversals during the period of record at ADCP3, so it is unclear whether the currents there are likely to reverse when the wind changes direction.
The response of the lake to wind stress under prevailing wind conditions (winds from the west to northwest) is illustrated conceptually in figure 8. Under these conditions, the currents over the broad, shallow part of the lake to the east of Bare Island flow down the lake to the southeast. Strong currents flowing northwest carry a return flow in the narrow trench area to the west of Bare Island. Circulation in the study area is clockwise under prevailing wind conditions. Preliminary modeling has indicated that most of the water in the lake turns around in the southern part of the lake between Howard Bay and Buck Island, but this part of the circulation is complicated and will require further study to refine (R.T. Cheng, U.S. Geological Survey, unpub. data, 2004). It is clear, however, that the pattern of a broad flow to the southeast on the eastern side of the lake and a narrow flow to the northwest through the trench on the western side of the lake is a persistent feature. When the wind reverses, the basic flow pattern reverses as well, to a broad flow to the northwest on the east of Bare Island, narrow flow to the southeast in the trench, and a counterclockwise circulation through the study area. The basic principles of the flow pattern were suggested in earlier modeling work (Blank and Juza, 2001), but the data to confirm and refine the results were not available until the placement of the ADCPs in the lake during this study.
An instantaneous measurement of the discharge northward through the trench west of Bare Island (cross-section A-B in fig. 8) obtained in June 2005 was 580 m3/s (R.E. Wellman, U.S. Geological Survey, written commun., 2005). On the basis of this value and a Geographic Information System (GIS) estimate of the volume of the lake north of Buck Island of 475×106 m3 (T.L. Haluska, U.S. Geological Survey, written commun., 2005), an estimate of the traveltime around the entire loop is on the order of 10 days. This is not to be confused with residence time in the lake, the amount of time between when a parcel of water enters the lake and when it leaves, which is likely to be many times longer.
U.S.
Department of the Interior | U.S. Geological
Survey
Persistent URL: https://pubs.water.usgs.gov/sir20065209
Page Contact Information: Publications Team
Page Last Modified: Thursday, 01-Dec-2016 19:12:59 EST
