Warmer and Drier Climates That Amake Terminal Great Lakes

Home , Lake Agassiz, Lake Nipigon, Mattawa River

J. Great Lakes Res. 32:852–869 Internat. Assoc. Great Lakes Res., 2006

Warmer and Drier Climates that Make Terminal Great Lakes

Thomas E. Croley II1,* and C. F. Michael Lewis2 1Great Lakes Environmental Research Laboratory National Oceanic and Atmospheric Administration 2205 Commonwealth Blvd. Ann Arbor, Michigan 48105-2945 2Geological Survey of Canada (Atlantic) Natural Resources Canada Bedford Institute of Oceanography Box 1006, Dartmouth, Nova Scotia B2Y 4A2 and Graduate School of Oceanography University of Rhode Island Narragansett, Rhode Island 02882

ABSTRACT. A recent empirical model of glacial-isostatic uplift showed that the Huron and Michigan lake level fell tens of meters below the lowest possible outlet about 7,900 14C years BP when the upper Great Lakes became dependent for water supply on precipitation alone, as at present. The upper Great Lakes thus appear to have been impacted by severe dry climate that may have also affected the lower Great Lakes. While continuing paleoclimate studies are corroborating and quantifying this impacting climate and other evidence of terminal lakes, the Great Lakes Environmental Research Laboratory applied their Advanced Hydrologic Prediction System, modified to use dynamic lake areas, to explore the deviations from present temperatures and precipitation that would force the Great Lakes to become terminal (closed), i.e., for water levels to fall below outlet sills. We modeled the present lakes with pre-development natural outlet and water flow conditions, but considered the upper and lower Great Lakes separately with no river connection, as in the early Holocene basin configuration. By using systematic shifts in precipitation, temperature, and humidity relative to the present base climate, we identified candidate climates that result in terminal lakes. The lakes would close in the order: Erie, Superior, Michigan- Huron, and Ontario for increasingly drier and warmer climates. For a temperature rise of TºC and a precipitation drop of P% relative to the present base climate, conditions for complete lake closure range from 4.7T + P > 51 for Erie to 3.5T + P > 71 for Ontario. INDEX WORDS: Climate change, Great Lakes, hydrology, water levels, terminal lakes.

INTRODUCTION Ojibway in northern Ontario and northeastern Que- bec, and drained into the Ottawa River valley and Background and Purpose thence to the North Atlantic Ocean via the St. About 9,500 radiocarbon (14C) years before pre- Lawrence River valley (Teller and Leverington sent (BP) the large upstream glacial Lake Agassiz 2004), thereby bypassing the Great Lakes basin; see in northwestern Ontario and Manitoba supplied Figure 1. melt water from the last glaciation to the upper Recent construction of an empirical exponential Great Lakes through outlets to the Lake Nipigon model of isostatic uplift for the Great Lakes region basin and thence to the Superior and Huron-Michi- following the last glaciation allowed comparison of gan basins (Teller 1985, 1987). About 8,000 14C the elevations of rebounding lake outlets with re- years BP, Lake Agassiz merged with glacial Lake constructed lake levels based on 14C-dated water level indicators such as abandoned shorelines, iso- lation basins, submerged tree stumps, and unconfor- *Corresponding author. E-mail: [email protected] mities (Lewis et al. in press a, b). The early

852 Climates Closing Great Lakes 853

FIG. 1. Paleogeography of the region north of the upper Great Lakes at 7,900 14C BP when the combined outflow of Lakes Agassiz and Ojibway was routed to the Ottawa and St. Lawrence river valleys (solid arrow). At this stage the water supply of the Great Lakes was no longer supplemented by inflow from upstream sources, but was supplied by precipitation alone, as at present. Prior to 8,000 14C BP, overflow from Lake Agassiz passed into the Lake Nipigon and Lake Superior basins (open arrow). The upper Great Lakes then overflowed the North Bay outlet (open arrow) into the Mattawa River (M) and thence to the Ottawa and St Lawrence river valleys. Adapted from Figure 4p in Teller and Leverington (2004).

Holocene results for the Huron, Georgian Bay, and The inferred 7,900 14C years BP low stand of the Michigan basins reveal several periods of low lake Michigan and Huron basins occurred after melt levels (lower than present) due to overflow water drainage from upstream glacial Lake Agassiz drainage through the isostatically depressed outlet began to bypass the Great Lakes basin, leaving it area near North Bay, Ontario, to the Ottawa and St. susceptible to the early dry Holocene climate (Ed- Lawrence valleys; see Figure 1. These results were wards et al. 1996). As the Great Lakes basin had anticipated on the basis of previous syntheses (Es- then entered its present hydrological regime of chman and Karrow 1985; Hansel et al. 1985; Lewis and Anderson 1989; Barnett 1992; Clark et al. water supply by precipitation only, and as differen- 1994; Colman et al. 1994a, b; Lewis et al. 1994; tial glacial-isostatic crustal uplift was accounted Rea et al. 1994; Moore et al. 1994, 2000; and Lar- for, the only known process that could explain the son and Schaetzl 2001). Surprisingly, however, the sub-outlet low levels is climatic reduction in water results showed that lake levels fell below the supply, either by enhanced evaporation or reduced Huron-Michigan basin outlet after 8,000 14C years precipitation or both. [This result for the Huron and BP, possibly for a few centuries during which low- Michigan basins is similar to the conclusions of est water levels were up to 30 m below the over- Holcombe et al. (2003) who recognized low-level flow sill at North Bay; see Figure 2. shore features beneath Lake Erie, and noting the 854 Croley and Lewis

creased lake water salinity and reduced precipitation at the time of the closed low stands (Sarvis 2000, Blasco 2001), conditions that are consistent with reduced water supply. In Hamilton Harbour, western Lake Ontario, studies of ostracodes (De- lorme 1996), diatoms, pollen, and isotopes (Duthie et al. 1996) all reveal a low water phase and sug- gest a drier climate about 7,500 14C years BP. Simi- larly, in Mud Lake on the Keweenaw Peninsula beside Lake Superior, pollen and plant macrofossil analyses indicate an onset of a drying episode at 7,900 14C years BP with notably reduced water levels extending to after 7,000 14C years BP (Booth et al. 2002). As these widely-separated sites both indicate a lowering of lake levels and onset of a drier climatic episode, it is reasonable to envision that the inferred dry climate conditions that impacted the Huron and Michigan basins also affected other basins of the Great Lakes system. The low stand period correlates also to a relatively rapid transition in vegetative cover in the Great Lakes region from bo- real to the mixed Great Lakes-St. Lawrence Forest as recorded in pollen diagrams (McAndrews 1994, FIG. 2. Huron basin lake level (black line) Dyke et al. 2004). Complementary review and as- between 10,000 and 7,000 cal BP based on the sessment of the available paleoclimate information original elevations of lake-level indicators com- for the Great Lakes watershed, coupled with new puted by an empirical exponential model of glacial studies of proxy climatic and limnological indica- rebound for the Great Lakes basin that removes tors (pollen, isotopes, diatoms, ostracodes, and the- the effects of glacial-isostatic uplift. For clarity, camoebians) focused on the period spanning the data are removed from the plot except for the closed low stand are in progress, and will be re- Stanley unconformity constraint (S). At about ported in future publications. 7,900 14C BP (about 8,800 cal BP) the lake level, For this re-assessment, it would be helpful to ob- indicated by the Stanley unconformity (S–line tain approximate information about the amplitude indicates original elevation and circles bracket its of climatic change that might be expected to have age of 7,900 ± 300 14C BP), descended tens of caused the upper Great Lakes low stands. Accord- meters below the North Bay outlet (gray band), the ingly, we have used a hydrological model to ex- lowest possible point of overflow for the Huron plore the excursion from the present climate that basin at the time. The thickness of the grey band would force the Great Lakes into hydrologic clo- indicates the depth of water over the outlet sill at sure in terms of increased temperature and reduc- full discharge. Adapted from Lewis et al. (in press tion in precipitation. In this study, the hydrology of a, b). the Erie and Ontario basins is considered separately from that of the upper Great Lakes, as the isostati- high level of evaporative losses in the present Lake cally-depressed North Bay outlet for the upper Erie water balance, suggested that the Erie lake lakes remained at a lower elevation than the St. level may have fallen below the level of its outlet Clair River connection to Lake St. Clair and Lake sill because of enhanced evaporation sometime in Erie until much later, about 5,500 14C years BP (Es- the early to middle Holocene.] This episode of low- chman and Karrow 1985). Other attributes of the est levels appears as an extraordinarily severe im- region such as land cover, geography, and bathyme- pact of a dry climate, possibly of short duration, on try were modeled as they are at present. the upper Great Lakes hydrological system and may This low stand episode offers an opportunity, have extended to the lower lakes. once paleoclimate is better quantified, to acquire in- Preliminary study of thecamoebians and pollen in formation about the hydrological sensitivity of the the sediment sequence of Georgian Bay suggests in- Great Lakes system to high-amplitude climate Climates Closing Great Lakes 855

change. Such information would be beneficial for model studies and projections of future levels of the Great Lakes under global warming in which some climate modeling scenarios project levels below instrumentally-observed “natural variability” (Mortsch and Quinn 1996, Mortsch et al. 2000, Lofgren et al. 2002). It should be noted that the 7,900 14C years BP low stand episode occurred while the residual Laurentide ice sheet, then in the latitudes of Hudson Bay, was rapidly retreating and wasting away (Dyke et al. 2003). This was a period of rapid change in the proportions of land, ice, and water areas, with parallel changes in albedo and re- organization of atmospheric circulation (Dean et al. 2002). As a result, the inferred occurrence of closed low stand conditions in the Great Lakes basin is seen as a product of extremely unusual conditions. It is not regarded as an analog for future conditions, FIG. 3. Laurentian Great Lakes location map. but rather, as a natural experiment from which im- portant information about lake-climate sensitivity might be derived. Superior, Michigan, and Huron water levels, ac- The purpose of this paper is to demonstrate that if cording to Regulation Plan 1977, under the auspices a climate is extreme enough, levels on some Great of the International Joint Commission. Lakes would drop sufficiently to cut off outflow, Lakes Michigan and Huron are considered to be thereby making those lakes terminal. We look at ex- one lake hydraulically because of their connection cursions in temperature and precipitation from the through the deep Straits of Mackinac. A relatively present climate to disclose those values that would small flow of Lake Michigan water is diverted into drive the Great Lakes hydrology to produce termi- the Mississippi River basin at Chicago. The water nal lakes. This is not an attempt to simulate past hy- flows from Lake Huron through the St. Clair River, drology exactly, but to explore the possible Lake St. Clair, and Detroit River system into Lake magnitude of changed climates that might have pro- Erie. The drop in water surface between Lakes duced terminal lakes about 7,900 14C years ago in Michigan-Huron and Lake Erie is only about 2 m (8 accordance with recently acquired glacial-isostatic ft). This results in a large backwater effect between rebound evidence that Huron and Michigan basin Lakes Erie, St. Clair, and Michigan-Huron; changes lake levels had descended below their overflow out- in Lakes St. Clair and Erie levels are transmitted lets. upstream to Lake Michigan-Huron. From Lake Erie, the flow is through the Niagara River and Welland Diversion (used for navigation Study Area and hydropower) into Lake Ontario. There is also a The Great Lakes basin area is 770,000 km2 small diversion into the New York State Barge (300,000 mi2), about one-third of which is water Canal System which is ultimately discharged into surface; see Figure 3. The basin extends 3,200 km Lake Ontario. Lake Ontario outflows and levels are (2,000 mi) from the western edge of Lake Superior regulated in accordance with Regulation Plan to the St. Lawrence Power Project, Cornwall, On- 1958D to balance interests upstream on Lake On- tario on the St. Lawrence River. The water surface tario with those downstream on the St. Lawrence drops in a cascade over this distance some 180 m Seaway. The outflows are controlled by the Moses- (600 ft). Lake Superior is largest and deepest and Saunders Power Dam between Massena, New York has two diversions into it: the Long Lac and Ogoki. and Cornwall, Ontario. From Lake Ontario, the Lake Superior flows through the lock and compen- water flows through the St. Lawrence River to the sating works at Sault Ste. Marie and down the St. Gulf of St. Lawrence and to the Atlantic Ocean. Marys River into Lake Huron where it is joined by Lakes Superior, Michigan, Huron, and Ontario are water flowing from Lake Michigan. Lake Superior very deep (229–405 m) while Lakes Erie and St. outflows and levels are regulated to balance Lakes Clair are very shallow (6–64 m). 856 Croley and Lewis

Approach We use the Great Lakes Environmental Research Laboratory’s (GLERL’s) Advanced Hydrologic Pre- diction System (AHPS), a system of hydrology, thermodynamic, and hydraulic models for the Great Lakes (Croley 2005). GLERL uses these models to make probabilistic outlooks of Great Lakes hydrology and water levels (see http://www.glerl.noaa. gov/wr/ahps/curfcst /curfcst.html), and to assess climate change impacts in the Great Lakes (Croley and Luukkonen 2003, Croley 2003, Lofgren et al. 2002, Quinn and Croley 1999, Croley et al. 1998). For the purpose of this study, we adjusted the present models to simulate the Great Lakes in their pre-European-settlement natural state by removing the influences of channel control works and regulation plans. Also, we kept watersheds of the upper and lower lakes separate, as they were during the early Holocene, i.e., no outflow from the Huron basin to the St. Clair-Erie basin. Accordingly, we use the models here with water balances on all lakes, and with lake outflow rating curves, selected to represent “natural” or “pre-development” conditions. We account for lake area variations with changes in water level, but do not remove present- day diversions and consumptions as they are relatively insignificant for our purpose. First we consider all lakes as interdependent (as they are now, but with “natural” outlet and connecting channel flows) to see if simulations with historical meteorology (1948–1999) produce hydrology FIG. 4. Great Lakes Meteorological Station Net- and lake levels comparable with the historical work—overland (top) and overlake (bottom). records. This allows us to assess the reasonableness of the modified models. Then, we model two sys- 1,800 stations for over-land meteorology (precipita- tems of Great Lakes independently: 1) Lakes Supe- tion and air temperature) and about 40 stations for rior, Michigan, and Huron (the upper Great Lakes), over-lake meteorology (air temperature, humidity, and 2) Lakes St. Clair, Erie, and Ontario (the lower wind speed, and cloud cover); see Figure 4. These Great Lakes) with no inflow from the upper Great data, compiled for previous studies (Croley 1990, Lakes since they drained via the North Bay outlet to Hartmann 1990, Croley 1992b, Croley et al. 1998, the Mattawa and Ottawa rivers when overflow oc- Lofgren et al. 2002, Croley 2003), provide daily curred prior to and after the low stand 7,900 14C meteorological time series over each of the 121 years ago. Next, we consider steady state hydrology riverine watersheds that drain into the Great Lakes by modeling over an extended period constructed and the seven Great Lake water surfaces. Annual by repeating the adjusted meteorological record average precipitation and air temperature are, re- until consecutive 52-year segments are identical. spectively, 80.9 cm and 2.93°C (Superior basin), We finally consider each lake as part of its parent 84.5 cm and 6.49°C (Michigan-Huron), 91.9 cm system (upper or lower system) with a water bal- and 9.19°C (Erie), and 92.3 cm and 7.41°C (On- ance on all lakes. tario). We used these historical meteorological data with our hydrology models (discussed subse- CHANGED-CLIMATE METHODOLOGY quently) to compute the “present” or “base case” The hydrology models here use daily meteoro- scenario. We then apply selected precipitation ratios logical data from 1948–1999, compiled from about and air temperature differences to the historical me- Climates Closing Great Lakes 857 teorological data and use these modified meteoro- between daily model outflows and adjusted outflow logical time series with our hydrology models to observations. Each calibration determined parame- construct changed climate scenarios. ters for infiltration, snow melt, surface runoff, per- All precipitation is adjusted by multiplying the colation, interflow, deep percolation, groundwater actual precipitation by a single precipitation ratio flow, surface storage, and evapotranspiration from and all air temperatures are adjusted by adding a all moisture storages by systematically searching single temperature difference to the actual tempera- the parameter space (with a gradient-search tech- tures. In addition, humidity is adjusted; for precipi- nique). The model agrees quite well with weekly tation ratios below unity, which are all that are and monthly outflow observations (Croley 2002, considered here, the absolute humidity is multiplied 2003). These parameters represent present-day hy- by the ratio. Thus, if precipitation is halved, then so drology and are not changed in the simulations. All is humidity. 121 model applications are used in the simulations.

HYDROLOGY MODELS Evaporation GLERL’s AHPS consists of daily runoff models GLERL’s Lake Thermodynamic Model adjusts for each of the 121 watersheds, lake thermody- over-land data (original or adjusted as a changed- namic models for each of the seven water bodies, climate scenario) from the 40 over-land stations hydraulic models for the four connecting channels that are used to estimate over-water meteorology and five water body outflow points with operating for over-water or over-ice conditions based on em- plans encoded for Lakes Superior and Ontario, and pirical relationships between the two (Croley 1989, simultaneous water balances on all the lakes. It is 1992a; Croley and Assel 1994). Surface flux described in detailed overviews elsewhere (Croley processes are represented for reflection and short- 2003, 2005). wave radiation, net long-wave radiation, and advec- tion. Aerodynamic equation bulk transfer Runoff coefficients for sensible and latent heat are formu- lated with atmospheric stability effects. Energy GLERL’s Large Basin Runoff Model (LBRM) conservation accounts for heat storage; superposi- consists of moisture storages arranged as a serial tion of heat additions or losses determines tempera- and parallel cascade of “tanks” coinciding with the ture-depth profiles. Each addition is parameterized upper and lower soil zones, a groundwater zone, by age and mixes throughout the volume. Mass and and the surface channel system (Croley 2002). energy conservation account for ice formation and Water enters the snow pack, which supplies the decay. The model has been calibrated to each of the basin surface (degree-day snowmelt). Infiltration is seven lake surfaces by minimizing root mean proportional to this supply and to saturation of the square error between daily model surface tempera- upper soil zone (partial-area infiltration). Water per- tures and observations. The model enables one-di- colates from the upper to the lower soil zone and mensional modeling throughout of spatially from the lower to the groundwater zone (deep per- averaged water temperatures over the lake depth colation). Water also flows from the upper, lower, and can be used to follow thermal development and and groundwater zones into the surface channel turnovers in the lake. system, as surface runoff, interflow, and groundwater flow respectively. “Groundwater” refers to intra-, not inter-, watershed storage. Flows from all Lake Area Adjustment tanks are proportional to their amounts (linear- For each lake, precipitation p is provided as a reservoir flows). Evapotranspiration is proportional scenario-dependent boundary condition and runoff r to available water and to sensible heat (a comple- and evaporation e are estimated with the runoff and mentary concept in that evapotranspiration reduces evaporation models. They are expressed as depths available sensible heat). Mass conservation applies over the lake surface, in m, for a given time interval for the snow pack and tanks; energy conservation (day), and are based on the lake area C as coordi- applies to evapotranspiration. Complete analytical nated between the US and Canada (CCGLBHHD solutions exist. The model has been calibrated to 1977). That is, no variation of lake area is actually each of the 121 watersheds contributing to the considered in their determination in the runoff and Great Lakes by minimizing root mean square error evaporation models. However, we adjust to actual 858 Croley and Lewis

lake area A by converting these depth rates into vol- cation of connecting channels through dredging or umetric flow rates, shoreline changes, use of ice control measures, and diversion of water into and out of the lakes. Any pA impacts caused by land use modification, consump- P = (1) ∆ tive uses, and regulation of tributary rivers are viewed as reflected by changes in water supplies to C BA− the lakes and not by changes in elevation—outflow Rr= (2) − ∆ relationships, and were not considered in that study. BC We converted Southam’s relationships from their original English units and IGLD’55 water level eA E = (3) datum (CCGLBHHD 1979) to metric units and ∆ IGLD’85 water level datum (CCGLBHHD 1995), respectively (Croley 2006). We also transformed his where P = volumetric precipitation rate in m3s–1, Lake Erie adjustment for channel project removals R = volumetric runoff rate in m3s-1, E = volumetric to one compatible with basic weir formulae and ex- evaporation rate in m3s–1, B = basin area (including pressed Ontario outflows in terms of the 1985 sill the lake), and ∆ = number of seconds in the time in- elevation (Croley 2006). The resultant equations are terval. Note, B and C are constants for a lake while p, r, e, and A vary with time. Precipitation and QZ=−−824.( 721 181 .) 425 15. H , Z ≥ 181. 425 (4) evaporation are directly converted by simply multi- SS SS plying the overlake rates by actual lake area. Runoff is first multiplied by the coordinated lake 2 =+1 1 −  area (over which it was expressed) to calculate the QZZTTC46.. 440 166 549 2 2  modeled runoff volume, then divided by the coordi- 1 −−2 ≥≥ nated land area (to express it as the equivalent yield ()ZZTC HT , ZZTC166. 549 per unit of land area), and then multiplied by the ac-  1 1  2 tual land area to calculate the adjusted runoff vol- =−46.. 440 Z 166 549  2 T 2  (5) ume. Thus “R” gets bigger as “A” gets smaller. Of 1 − 2 − ≥> course, there is some error involved with this proce- (.ZT 166 549)),HT ZZTC166. 549 dure since p, r, and e actually depend on actual lake area too and should have been computed from models considering actual lake area and volume =−2 changes. Also, exposed land areas would not have QZCC70.( 714 165 .) 953 the same properties as the original basin. Consider- 1 (),ZZ−−2 H ZZ≥≥165. 953 ation of the uncertainty associated with these errors CE C CE = − 2 is beyond the scope of this exploratory study. 70. 714(.)ZC 165 953 (6)

1 −−2 ≥> (.)ZHCC165 953 ZZCE165. 953 Outﬂow Relations Unmanaged lake outflow depends on lake level =−−15. ≥ and outflow sill elevation for lakes not affected by QZEE701.( 504 169 .) 938 H E , ZE 169. 938 (7) backwater (such as Superior, Erie, and Ontario) or on these variables as well as downstream lake level for lakes affected by backwater (such as Michigan- =−−15. ≥ (8) QZOO577.( 187 69 .) 622 H O , ZO 69. 622 Huron and St. Clair). (We consider the present Great Lakes here to facilitate later validation of the model.) Southam (1989) described a quantitative where QS, QT, QC, QE, and QO = outflows from empirical relationship between water elevation and Lakes Superior, Michigan-Huron, St. Clair, Erie, 3 –1 outflow for each lake that represents “natural” con- and Ontario, respectively in m s , ZS, ZT, ZC, ZE, ditions, prior to the introduction of societal devel- and ZO = respective water elevations with respect to opments. For the Laurentian Great Lakes the IGLD’85 water level datum in m, and HS, HT, watershed, these developments include regulation HC, HE, and HO = respective ice retardations in of outflows of Lakes Superior and Ontario, modifi- m3s-1 as shown in Table 1. Note, outflows are zero Climates Closing Great Lakes 859

TABLE 1. Great Lake outflow ice and weed when the Superior water level is below the sill of a retardation (Southam 1989). 181.425m, QT = 0 when Michigan-Huron is below the sill of 166.549 m, Q = 0 when St. Clair is Month Superior Mich.-Huron St. Clair Erie C m3s-1 m3s-1 m3s-1 m3s-1 below the sill of 165.953 m, QE = 0 when Erie is (1) (2) (3) (4) (5) below the sill of 169.938 m, and QO = 0 when On- tario is below the sill of 69.622 m. January 113 1,020 425 113 Since (4)–(8) were derived from semi-empirical February 113 136 425 142 March 113 651 227 85 stage-fall-discharge or rating curves that were fit to April 113 170 57 142 a range of flows and elevations not necessarily May close to the sill, the sill elevations estimated here June 57 are in error. Sill heights on all lakes but St. Clair are July 142 well above the bottom of the lake. On Lake St. August 113 Clair, the bottom of the lake is 168.4 m (subtract September 85 maximum coordinated depth from chart datum in October 57 column 6 of Table 2); this is above the Michigan- November Huron and St. Clair sills. This corresponds to a December 113 142 channel running along the bottom of Lake St. Clair; aNo values for Ontario are given in the reference. i.e., the lake bottom is at the top of this channel and we can have flow from the Lake St. Clair basin without lake storage. Since the lake bottom is when elevation is below the “sill” elevation of the below the Erie sill of 169.938 m, we see that St. lake; the sill is the lowest elevation for which flow Clair will never be empty as long as Lake Erie is from the lake is still possible (e.g., Lake Superior’s not terminal (water line above its sill). Lake out- sill level is 181.425 m). We ignore the small eleva- flows in (4) and (5)–(8) are set to zero when nega- tion differences, introduced by the datum change, tive values would be computed (ice retardation between Michigan-Huron and St. Clair levels and would drop to equal flow rate). between St. Clair and Erie levels to keep the equations physically meaningful; i.e., when Lakes Hypsometric Relations Michigan-Huron and St. Clair are at the same level The Coordinating Committee on Great Lakes (ZT = ZC) or Lakes St. Clair and Erie are at the Basic Hydraulic and Hydrologic Data (CCGLB- same level (ZC = ZE), there should be no flow be- HHD 1977) provided graphical relations, for each tween the respective pair of lakes (QT = 0 or QC = lake, between depth and volume; inspection reveals 0). However, backflow is possible from Lake Erie that simple power relations are a very good fit, to Lake St. Clair and from Lake St. Clair to Lake Michigan-Huron. This backflow is not described by VaMD=−()b these equations (but is addressed subsequently). d − (9) Note that when St. Clair water level is below the A =−VabMD =() − b 1 dD Michigan-Huron sill, the sill elevation is controlling in (5); likewise when Erie water level is below the where A = area of horizontal surface at depth D St. Clair sill, the sill elevation is controlling in (6). below a reference elevation, M = maximum depth, These are reasonable extensions, made here to V = lake volume beneath horizontal surface at depth allow flow computations as lake levels drop below D, and a and b are empirical parameters. By requir- those historically experienced. Note that QS = 0 ing that the coordinated values of area, C, and vol-

TABLE 2. Coordinated values of Great Lake parameters (CCGLBHHD 1977). Parameter SUP MIC HUR GEO STC ERI ONT (1) (2) (3) (4) (5) (6) (7) (8) chart datum, m 183.2 176.0 176.0 176.0 174.4 173.5 74.2 maximum depth, m 405 281 229 164 6 64 244 coordinated area, km2 82,100 57,800 40,640 18,960 1,114 25,700 18,960 coordinated volume, km3 12,100 4,920 2,761 779 3.4 484 1,640 860 Croley and Lewis

ume, S, (CCGLBHHD 1977) exist at the reference CC elevation (chart datum), where D = 0, for each lake, VIQCCC( )() pAR CCC BACC eA CC(15) ∆∆≅− + + BC −− as in Table 2, the parameters are CC−

C CE aM VIQEEE( )() pAR EEE BAEE eA EE(16) = S ∆∆≅− + +BC −− (10) EE− S b = M a CO ∆∆VIQOOO≅−( )() + pAR OOO + BAOO−− eA OO(17) Writing (9) in terms of elevation instead of depth, BCOO− where V = change in volume and the subscripts VaZZ()b ∆ =−B (11) refer to individual Great Lakes or extended water b−1 AabZZ=−()B bodies: Superior (S), Michigan (M), Huron (H), Georgian Bay (G), Michigan-Huron (T), St. Clair (C), Erie (E), and Ontario (O). Equation (14) con- where Z = elevation at depth D, in m, and ZB = elevation of lake bottom, in m, given from Table 2 by siders Lakes Michigan and Huron, including Geor- subtracting maximum depth from chart datum. gian Bay, as one water body. Boundary conditions are Water Balance IS = 0 (18) The adjusted over-lake precipitation, runoff to the lake, and lake evaporation are used in a water balance, IQTS= (19) dV IQPRE (12) dt = −++− IC = QT, for upper Great Lakes flowing into lower = 0, for upper Great Lakes flowing into the (20) where t = time, I = volumetric water body inflow Mattawa and Ottawa basins rate (outflow from the upstream lake) , and Q = volumetric water body outflow rate. Note that V, A, IQ (21) and Q are not simple functions of Z. Determination EC= of the proper equation to use in (4)–(8) depends on downstream water level, which in turn depends on IQ (22) which equation is used. Likewise, backwater adjust- OE= ments (described subsequently) are not reflected in (4)–(8). For these reasons, it is not possible to solve For each water body, it is necessary to compute (1)–(12) analytically. Equations (1)–(3) and (12) are the inflow as outflow from the upstream lake, the applied over time interval ∆ to each water body lake(s) area, and the adjusted net basin supplies as based on the lakes and connecting channels part of the solution. This requires calculating lake arrangement, levels as part of the water balance. We solve (4), (5)–(8), (11) for each lake, (13)–(17), and (18)–(22) CS VIQSSS( )() pAR SSS BASS eA SS(13) simultaneously at each time step. Our numerical ∆∆≅− + +BC −− SS− procedure at each time step is: i) given p, r, e, and Z (water elevation at beginning of time step) for all ()()VVV IQ 0 ∆∆MHG++ ≅ T − T lakes, ii) calculate A0 (lake area at beginning of C time step) and V (lake volume at beginning of time pA R M (B AeA) 0 ++MM MBC M −−−MMM step) for all lakes from (11) and Q (outflow rate at MM− (14) 0 beginning of time step) for all water bodies from CH ++pAHH R H ()BAHH−− eA HH (4) and (5)–(8), iii) approximate Z1 (end-of-time- BCHH − step water elevation) as Z0 for all lakes, iv) calcu- CG late A1 (end-of-time-step lake area) for all lakes + pGGAR+ G ()BAGG−− eA GG BCGG− from (11) and Q1 (end-of-time-step water body out- Climates Closing Great Lakes 861

flow rate) for all water bodies from (4) and (5)–(8), v) approximate outflow rates and lake areas over the time increment as linear, Q = (Q0 + Q1)/2 and A = (A0 + A1)/2, vi) calculate the changes in storage for all water bodies over the time interval by using these approximate outflow rates and lake areas in (13)–(17) and (18)–(22), and vii) calculate V1 = V0 + ∆V for each lake and then find Z1 by using V1 with (11) for each water body (for Lake Michigan- Huron, interpolate for Z1 by using V1 with (11) applied to Lakes Michigan, Huron, and Georgian Bay and summed). Repeat steps iv–vii until successive values of Z1 for all lakes change negligibly. Repeat steps i–vii for the next time step, and so forth. When solving (4), (5)–(8), (11), (13)–(17), and (18)–(22), we check and correct for backflow between lakes. This could occur if water levels on Lake Erie are above those on St. Clair (and above the St. Clair sill) or those on St. Clair are above those on Lake Michigan-Huron (and above the Michigan-Huron sill). For those times when backflow would occur between two lakes, we simply balance the lakes involved so that water levels on both are equal and the flow between them is zero. Furthermore, we consider sill heights in this adjustment and do not let backflow reduce a lake’s level below the upstream sill. Note that backflow does not occur when simulating the existing system with the existing climate. It also does not occur when simulating the upper lake system (Superior, Michi- gan, and Huron) with any climate since (5) is re- placed with a relation that is a function of Michigan-Huron levels only (discussed subse- FIG. 5. Net basin supply comparison for 1948- quently). Backflow corrections are only required 1999 showing reasonable agreement between when simulating the existing system or the lower water supplies based on computation of observed lake system (St. Clair, Erie, and Ontario) with lake levels and flows (historical) and supplies sim- warmer or dryer climates. The equations solution ulated from observed meteorology (modeled). converges to an insignificant difference within 2–15 iterations (the difference between water elevations els and flows). Figure 5 compares our estimates in successive iterations, summed over all lakes, is with historical NBS and shows good agreement, as less than one thousandth of a millimeter). expected since historical meteorology data are used in the simulation. Differences can be ascribed to VALIDATION water balance errors in the computation of residual To check the models and water balance approxi- NBS and to modeling errors in the computation of mations, we simulated the entire interconnected the NBS components. The biggest differences occur Great Lakes for the historical meteorological on Lake Ontario, suggesting they arise from water record. First, we compared simulated net basin sup- balance errors in computing the historical residual plies (precipitation + runoff – lake evaporation) re- NBS. sulting from the model, applied to the historical Next, we compared simulated lake levels result- meteorological record with actual initial conditions, ing from the model, applied to the historical meteo- directly to historical net basin supplies (computed rological record with actual initial conditions, as a water balance residual from historical lake lev- directly to historical levels. For this comparison, we 862 Croley and Lewis

ering probably results from the historical changes in Lake Superior operations and in the St. Clair River channel which has been dredged over time. It also may be related to variation in crustal rebound oc- curring after retreat of the last ice sheet; crustal rebound results in relative tilting of Lake Michigan-Huron toward its outlet suggesting higher outflows and lower levels in the historical record than simulated (Quinn and Sellinger 1990). Lakes St. Clair and Erie are very similar to the simulation but Ontario shows lower water levels historically, probably as a result of the difference between regulated Niagara flows and the natural outflow and channel conditions. The model appears to simulate the system reasonably well when all sources of differences between the simulations and historical flows are considered. Connecting channel flow differences (not shown here) also match well. FIG. 6. Great Lake levels as observed 1948-1999 (historical) compared with simulated levels (modeled) showing reasonable agreement. CHANGED CLIMATES Before applying the simulations to changed climates (i.e., changed temperature and precipitation), included all diversions but used the natural outflow we ascertained that the present-day diversions in and channel relationships. Figure 6 is a plot of daily the hydrology models were on the order of a few simulated levels and monthly historical levels; it shows fair agreement, but has expected deviations. centimeters; see Table 3 (IGLDCUSB 1985). [Note On Superior, levels match well with historical data that these diversions affect lakes upstream as well after about 1965 but differ before; this could be due as downstream. The Chicago diversion affects Su- to sparse water level station networks prior to 1965 perior because resultant lower Michigan-Huron lev- (hard to evaluate), poorer meteorological estimates els are used in regulation of Superior. The Welland prior to 1965 (when station densities are lowest on diversion lowers Lake Erie and, because of con- Superior and areal estimates are often underesti- necting channel hydraulics (upstream and down- mated), and differences in the outflow and channel stream lake levels determine channel flow), lower relationships (water was released on Superior in Erie levels lower Michigan-Huron and lower 1965 to alleviate low water levels downstream; Michigan-Huron levels lower Superior as just dis- there were also changes in the Superior regulation cussed.] Thus, they are negligible compared to the plan between 1970–77; the model simulation uses changes in net basin supplies or drops in water lev- an unchanging outflow and channel relationship). els to be simulated with changed climates. There- On Michigan-Huron, it appears that the historical fore, we ignore them; no effort was made to remove water levels are lower than the simulated; this low- these diversions from the existing models.

TABLE 3. Summary of average annual Great Lakes diversion impacts (IGLDCUSB 1985). Diversions Amount Superior Mich.-Huron Erie Ontario (m3s-1) (cm) (cm) (cm) (cm) (1) (2) (3) (4) (5) (6) Ogoki-Long Lac 160 +6 +11 +8 +7 Chicago 90 –2 –6 –4 –3 Welland 270 –2 –5 –13 0 COMBINED — +2 –1 –10 +2 Climates Closing Great Lakes 863

Steady-State Simulation the steady-state average water level resulting from We use both the historical and modified meteoro- each and plotted it with precipitation drop and tem- logical time series with our models to simulate base perature rise as shown in Figure 7 for Lakes Supe- case and climate change hydrology scenarios, re- rior and Michigan-Huron. Note three regions in spectively. We estimate steady state hydrology by each graph of Figure 7: the region where all water modeling with arbitrary initial conditions (snow levels are above the sill elevation in the lower left pack, water storages in the basins, thermal structure of the graphs, the region where all water levels are of the lakes, lake levels, and so forth) over an ex- below the sill elevation in the upper right of the tended period constructed by repeating the adjusted graphs, and the intermediate region where water meteorological record until consecutive 52-year levels are both above and below the sill elevation. segments are identical. (The models always con- We determined the boundaries of these regions by verge no matter where started). The number of iter- looking at maximum and minimum water levels in ations required to reach this state depends largely each simulation and comparing them to the sill ele- on the arbitrary lake level assumed at the beginning vations. Since behavior of steady-state average and the final lake levels; it sometimes represents a water levels is fundamentally different in each of longer time than might be expected for the climate these regions, we restricted linear interpolation in change itself. (The effect of the initial conditions each region to only values therein. other than lake levels is much shorter, usually on By using linear approximations, note that the cli- the order of a couple of years.) However, since lake mate isolines for a terminal Lake Superior in Figure levels are unknown prior to the changing climate 7 drop about 1°C for every 4.7% change in precipi- and since we want to avoid representing climate tation. Figure 7 suggests that Lake Superior should change as abrupt, we use this “steady-state” behav- be a terminal lake for climates with a temperature ior to assess the effects of climate change. rise T (°C) and a precipitation drop P (%) such that 4.7T + P > 60. Likewise, the isolines in Figure 7 for Upper Great Lakes a terminal Lake Michigan-Huron drop about 1°C Separating upper lakes (Superior, Michigan, for every 4.5% change in precipitation; Lake Michi- Huron) from lower lakes (St. Clair, Erie, Ontario), gan-Huron should be terminal for climates with a for purposes of simulating the system about 7,900 temperature rise T (°C) and a precipitation drop P 14C years BP, is accomplished by changing Lake (%) such that 4.5T + P > 63. Michigan-Huron outflow to a function only of the water level in Lake Michigan-Huron (and not of St. Lower Great Lakes Clair) and then by modeling only these upper lakes. The outflow function ideally should represent con- For the lower Great Lakes, we looked at Lakes ditions of 7,900 14C years ago but those are as yet St. Clair, Erie, and Ontario, with no inflow to Lake unknown. As a proxy, the spillway equation of (4), St. Clair as was the case prior to and after the low (7), or (8) was arbitrarily used along with the pre- stand 7,900 years ago when Michigan-Huron sent-day sill elevation taken from (5). We deter- flowed into the Mattawa and Ottawa watersheds; mined the leading coefficient by trial and error to see (20). Since the St. Clair lake bottom is above its match a long-term water balance with historical sill elevation, there can be flow into Erie even when levels, Lake St. Clair is empty. Thus, St. Clair can never be terminal (with water still in it); it can only dry up. We have to consider Lakes St. Clair and Erie as one QZ=−185( 166 . 549 )15. − HZ , ≥ 166 549 (23) . TT TT water body to investigate Lake Erie becoming terminal. We looked again at the 36 climate scenarios, Note again, outflow in (23) is set to zero for nega- previously defined, and calculated the steady-state tive values or for elevations below 166.549 m. water levels resulting from each. We found that When lake levels are always below the sill eleva- Lake Erie became terminal in this range of climate tion, then the lake is terminal. We looked at 36 cli- variations but Lake Ontario did not. Therefore, we mate scenarios, each defined in terms of the considered a larger range of climate variations by precipitation drop from the base case (0–50% in taking nine precipitation ratios (0–80% in steps of steps of 10%) and the temperature rise above the 10%) and eleven temperature rises (0–10°C in steps base case (0–5°C in steps of 1°C). We calculated of 1°C) and plotted the average steady-state water 864 Croley and Lewis

FIG. 7. Steady-state upper Great Lakes average water levels as a function of temperature rise and precipitation drop relative to the present base climate.

FIG. 8. Steady state lower Great Lakes average water levels as a function of temperature rise and precipitation drop relative to the present base climate. Climates Closing Great Lakes 865

level resulting from each in Figure 8 for Lakes Erie precipitation amounts, without regard to season of and Ontario. the year, by a constant ratio and we added to all his- Note that we again define three regions in each torical daily air temperatures, again without regard graph of Figure 8 for water levels above the sill, to season of the year, a constant value. Undoubt- below the sill, and both above and below the sill, by edly, we could consider more reasonable changes looking at maximum and minimum water levels and by considering the season of the year, and even lo- sill elevations. We again restrict linear interpolation cation. Again, as more is learned about past cli- in each region to only values in that region. Note mates from paleoclimatic considerations, we can that the isolines for a terminal Lake Erie in Figure 8 make these additional considerations. However, we drop about 1°C for every 4.7% change in precipita- think these results are generally indicative of how tion. Figure 8 suggests that Lake Erie should be a climate effects would influence Great Lakes termi- terminal lake for climates with a temperature rise T nal lake status. Indeed, Lofgren et al. (2002) sum- (°C) and a precipitation drop P (%) such that 4.7T + marized many of the past Great Lake climate P > 51. Likewise, the isolines in Figure 8 for a ter- studies that used general circulation model experi- minal Lake Ontario drop about 1°C for every 3.5% ments for 2×CO2 studies; those climates that were change in precipitation; Lake Ontario should be ter- warmer and drier showed good agreement with Fig- minal for climates with a temperature rise T (°C) ure 7. and a precipitation drop P (%) such that 3.5T + P > Since we used only the available 52 years of 71. daily meteorological data, continuously repeated, to represent steady-state, we biased our results some- SENSITIVITIES what; only the storm events on record are represented. The “transitional zone” in both Figures 7 Each climate considered herein is specified over and 8 might be wider if a longer period were used the entire upper Great Lakes basin or over the entire since more marginal storm events would be in- lower Great Lakes basin in their respective analy- cluded that allowed some small outflow at water ses. That is, the same changes made to historical levels close to sill elevations. data, to construct a hypothetical climate, were used There are also many errors of approximation in across all water bodies and their basins in each this study; our calculations used over-lake precipi- analysis. For example, the 1°C increase applied to tation, over-lake evaporation, and runoff to the lake Lake Ontario meteorological data was applied at from models that assumed fixed values (coordi- the same time to the Lake Erie meteorological data nated between the U.S. and Canada) for lake areas in the analyses. Thus, no consideration is made of and volumes, and then adjusted them for the actual more complex changed climates (such as a 1°C lake and basin areas obtained in a comprehensive change in Lake Erie air temperatures with a 2°C water balance. Better consideration would modify change in Lake Ontario air temperatures). Given the runoff and lake evaporation models directly to this limitation, the order of the lakes going terminal consider the actual lake areas and volumes in an in- as climate gets warmer and drier is approximately: tegrated water balance that employs these models Erie, Superior, Michigan-Huron, and Ontario. (The directly. Likewise, the hypsometric relations and order varies a little depending on the path of the outflow relations (both rating coefficients and sill changes from the present climate taken in Figures 7 elevations) could be improved. Different sill eleva- and 8.) For both Great Lake subsystems (upper and tions would shift the “terminal” lines in Figures 7 lower), the uppermost lake goes terminal before the and 8. lowermost lake; this is not strictly necessary. There Finally, the results do not exactly represent past may be climates (where meteorological conditions hydrology (for example, paleo-lake areas have not are different over the uppermost lake and the lower- been incorporated) so that results should be inter- most lake) that would yield the lowermost lake ter- preted as exploration on the question of what could minal while the uppermost lake was not terminal. be reasonably envisioned as the effect of various However, those changed climates were not investi- climate scenarios on the hydrology of the pre-de- gated herein. As more is learned about past climates velopment Great Lakes. This is an attempt to study from paleoclimatic considerations, we can fine tune the question of “What magnitude of drying and the observations made herein. warming of the present climate might produce ter- Likewise, the climate changes considered herein minal lakes as a guide to possible climate that ap- were simplified. We multiplied all historical daily parently produced hydrologic closure of at least 866 Croley and Lewis

some of the Great Lakes about 7,900 14C years water balances of the lakes and their connecting BP?” channels, lake area adjustments relating supplies It is possible that other aspects of climate may (lake precipitation, runoff, and evaporation) to the have been a factor in lowering lake levels about water balance, models of natural outflows and 7,900 14C years BP. In a study of the oxygen iso- channel flows, present-day hypsometric relations, topic composition of inorganic carbonate, cellulose and a water balance of all lakes and connecting from fossil wood, and lake sediments in southern channels. We tested the integrated model with his- Ontario, Edwards et al. (1996) show significant in- torical meteorological data (1948-1999) and found creases of mean annual temperature and summer it to be a reasonable model of Great Lakes water relative humidity from 8,000 to 7,000 14C years BP. levels. We built alternate climates from the histori- The early part of this interval is characterized as cal meteorological record by reducing precipitation cold and dry; only the latter part is interpreted as by fixed ratios and increasing temperature by fixed warm and dry relative to the present climate. West increments. We applied the integrated hydrology of Lake Superior, a study of Lake Ann sediments model to these alternate climates, producing associ- adjacent to a Holocene sand dune field showed that ated alternate lake level time series. The applica- dune (wind) activity started about 8,000 14C years tions were made separately in the upper and lower BP and was associated with relatively severe Great Lakes basins as overflows from the upper drought conditions (Keen and Shane 1990). Thus lakes prior to and after 7,900 14C years BP were future paleohydrological modeling focused on the routed via the Ottawa and St. Lawrence rivers, by- 7,900 14C years BP Great Lakes low stand may passing the Erie and Ontario basins completely. need to explore potential impacts under cold/dry The changed climate scenarios used in this study and windy conditions. Likewise, insolation would were simple: spatially and temporally constant ad- have been different at the time of the low stands justments were applied to historical meteorology than at present. For example, at 9, 000 14C years BP for each watershed and lake surface to estimate the average northern hemisphere summer solar radi- changed-climate meteorology for each watershed ation was up to 30 W m-2 greater and in winter it and lake surface. More complex climate change was a similar amount lower that at present considerations in our study of terminal Great Lakes (Kutzbach and Web 1993). wait on improved paleoclimatic reconstructions. Our results are biased by the length of the historical meteorology record we used. Errors of approxima- SUMMARY tion include linear adjustment of supplies for lake A new empirical model of glacial rebound and area, power equation hypsometric relations, and ap- comparison of past lake level indicators with outlet proximation of natural flow conditions and sill ele- elevations showed that lake levels in the Huron and vations for each Great Lake. Michigan basins had fallen below their outlets We modeled each alternate climate by repeating about 7,900 14C years BP (Lewis et al. in press a, our 52 years of adjusted meteorology until there b). As glacial-isostatic depression of outlets was ac- were no changes, in an effort to simulate steady- counted for, the only alternate known process that state conditions. It appears that Lake Superior could close the lakes is enhanced evaporation, re- would be a terminal lake if precipitation dropped duced precipitation, or both, in a dry climate. These 60% or more from the present or if air temperature findings motivated us to explore temperature and increased 60/4.7 = 13°C or more above the present precipitation excursions of the present climate that or some linear combination of the two, 4.7 + P > 60 might close the Laurentian Great Lakes as a guide where T and P are temperature rise (°C) and precip- to better understanding possible conditions at 7,900 itation drop (%), respectively. Likewise, it appears 14C years BP. We demonstrate the possibility that Michigan-Huron would be a terminal lake for P > changed climates could produce terminal Great 63% or T > 14°C or 4.5T + P > 63. Erie would be a Lakes by using present hydrology with natural (pre- terminal lake for P > 51% or T > 11°C or 4.7T + P development) channel and outflow conditions. We > 51. Ontario would be a terminal lake for P > 71% first integrated existing comprehensive models for or T > 20°C or 3.5T + P > 71. present-day large basin runoff applied to each of the Our study addresses only the question of climate 121 watersheds draining into the Great Lakes, mod- change necessary to close the pre-development els of present-day large-lake thermodynamics ap- Great Lakes and does not represent past or present plied to the seven water bodies of the Great Lakes, hydrology. We endeavored not to exactly model the Climates Closing Great Lakes 867 hydrology of the lakes around 7,900 14C years BP, Hydraulic and Hydrologic Data (CCGLBHHD) 1977. but to explore the potential magnitude of excursions Coordinated Great Lakes physical data. Physical in temperature and precipitation that could cause Data Subcommittee, Coordinating Committee on Great Lakes Basic Hydraulic and Hydrologic Data, the lakes to drop so low as to become “terminal” U.S. Army Corps of Engineers, Detroit District, lakes (with no outflow). Additional modeling could Detroit, Michigan. be done to accommodate the paleogeographic con- ——— . 1979. Establishment of international Great Lakes ditions of 7,900 14C years BP, such as incorporating datum (1955), 2nd Edition. Coordinating Committee changes in vegetation including those associated on Great Lakes Basic Hydraulic and Hydrologic Data, with changed lake areas. In addition, future model- U.S. Army Corps of Engineers, Detroit District, ing of the impacts of cold/dry and windier condi- Detroit, Michigan. tions as well as changed insolation are likely to be ——— . 1995. Establishment of international Great Lakes useful in understanding the Great Lakes low stands. datum (1985). Appendix A—Tabulation of primary bench mark elevations at gauge sites. Coordinating Committee on Great Lakes Basic Hydraulic and ACKNOWLEDGMENTS Hydrologic Data, U.S. Army Corps of Engineers, We are grateful for support of this study by the Detroit District, Detroit, Michigan. Paleoclimate program of the National Science Colman, S.M., Forester, R.M., Reynolds, R.L., Sweet- Foundation, grant ATM-0354762, and by the kind, D.S., King, J.W., Gangemi, P., Jones, G.A., CC4100 Climate Change program of the Earth Sci- Keigwin, L.D., and Foster, D.S., 1994a. Lake-level ences Sector of Natural Resources Canada. We are history of Lake Michigan for the past 12,000 years: the record from deep lacustrine sediments. J. Great pleased to acknowledge colleagues, G.R. Brooks Lakes Res., 20:73–92. and B.J. Todd of the Geological Survey of Canada ——— , Clark, J.A., Clayton, L., Hansel, A.K. and and J.W. King of the University of Rhode Island, Larsen, C.E. 1994b. 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