Currents

What can we learn from tracking currents in the Western Durham nearshore?

Water currents travel in a counter-clockwise direction around Lake Ontario. This is due to the Coriolis Effect and it occurs because the Earth rotates. By the equator, objects would have to move faster (and further) to complete the same rotation as objects near the Poles. So even though the entire lake is rotating with the Earth, the water at the southern end moves faster to keep up. North of the equator, surface waters are deflected to the right of the wind direction.

This means that if the wind is blowing from the North, a Southerly current forms and the water due to Coriolis moves right and the current will head down the West shore of the lake. Because the Great Lakes are so big, the water has room to turn through a complete circle, and this circular path is generally counter-clockwise. Once water is moving in lakes, it will follow a certain path which depends on the latitude of the lake and the speed the water is moving.

Lake Ontario Waterfront nearshore monitoring currents
Figure 1: The coriolis effect explained on a) a large scale, and b) within Lake Ontario. These images were created with the use of symbols courtesy of the Integration and Application Network, University of Maryland Center for Environmental Science (ian.umces.edu/symbols/)

The circulation patterns that occur in surface waters are called gyres, which are like large swirls. Wave action and shifts in prevailing wind direction can affect gyres. In Lake Ontario, there are actually two gyre patterns. The main circulation pattern for the lake is cyclonic and moves counter-clockwise as shown in the image above, however, in the northwest portion in the lake, there can also be a small clockwise pattern particularly in the summer (Beletsky et al., 1999).

Currents can be split into two components:

  1. Alongshore currents, which travel along the shoreline.
  2. Cross-shore currents, which move towards and away from the shoreline (on-shore/offshore).

In Western Durham, alongshore currents generally travel east and west, while cross-shore currents travel north and south. Currents do not travel exclusively in one direction; they have frequent flow-reversals. This means that sometimes the alongshore currents will be flowing west, and then they can reverse and flow east. In Lake Ontario, these flow-reversals generally occur every 3-5 days.

Lake Ontario Waterfront Nearshore Monitoring currents
Figure 2: Examples of the directions of Alongshore and Onshore-Offshore currents in the Western Durham region. “Map Source: Data provided by Ontario Ministry of Natural Resources © Copyright: 2005 First Base Solutions Inc. All Rights Reserved.

The Ontario Ministry of the Environment and Climate Change uses instruments called Acoustic Doppler Current Profilers (ADCPs; Figure 3) to measure the direction and the velocity of the currents in the nearshore.

Lake Ontario Waterfront nearshore monitoring currents
Figure 3: Image of ADCPs (Photo credit: Great Lakes Unit, EMRB, MOE, 2009).

If we look at the data that the ADCPs collect, we see that the alongshore currents have a greater velocity than the cross-shore currents. This means that the water in the nearshore region of Western Durham hugs the shoreline and flows east and west faster than it flows towards and away from the shore.

Lake Ontario Waterfront nearshore monitoring
Figure 4: Example of how to interpret ADCP data collected in Western Durham region. The months of the year are along the bottom of the graph, while the current velocities are up the side of the graph.

To put these graphs into context, we can estimate that it takes approximately 11 hours for the water to flow from Duffins Creek to Carruthers Creek when the alongshore velocity is 10 cm/s; this is about 0.4 km/hr. If the on-shore/offshore current is moving at 1 cm/s, then it will take approximately 1.5 days for the water to move the 4 km (at 0.04 km/hr).

We can see the influence of alongshore currents from many types of data. Using the ADCP data, we see that water moves much faster alongshore in the nearshore environment:

Lake Ontario Waterfront nearshore monitoring currents
Figure 5: Alongshore and Cross-shore current velocities southeast of Duffins Creek in 2008. Data collected by the Ontario Ministry of the Environment, and processed by TRCA.

Alongshore currents can also be seen with aerial photography, where we see the discharge from Duffins Creek traveling along the shoreline east towards Carruthers Creek.

Lake Ontario Waterfront nearshore monitoring currents aerial photograph
Figure 6: Alongshore transport observed in an aerial photo. Notice the darker area close to the shoreline illustrating that the currents are moving east.

Similarly, if we interpolate conductivity maps from cruises that the Ontario Ministry of the Environment and Climate Change conducted, we also see that the currents move the water along the shoreline. Conductivity is a great indicator of land-based influences. Interpolation is a technique that uses high resolution data and estimates what the values would be in between the measured data points. This can be used to create the maps below.

Lake Ontario Waterfront nearshore monitoring currents maps
Figure 7: Spatial conductivity maps show alongshore currents flowing in a) east, and b) west directions, in May and November, 2008 respectively. Data courtesy of the Environmental Monitoring and Reporting Branch, Ontario Ministry of the Environment.

Currents and Water Quality

If we look at the water chemistry data that the TRCA has collected in the nearshore, we see that the concentrations are highest by the shoreline, and they decrease as we move towards open waters and also as we move east or west of the nutrient source. The nutrients in nearshore regions are eventually diluted by offshore waters. This does not happen quickly or continuously though, it happens episodically. There are periods where alongshore flows hold nutrients from the rivers and shoreline close to the shore with brief periods of mixing with the offshore waters. These mixing periods are often driven by changes in the physical dynamics of the lake which result from changes in weather.

Lake Ontario Waterfront nearshore monitoring currents
Figure 8: TP concentrations from the shoreline to 3 km from shore for transects between Duffins (Transect 4) and Carruthers (Transect 1) Creeks on June 17, 2008. Symbol size decreases as you move from west to east.

In the example above, the red squares represent the transect by Duffins Creek; the concentrations decrease as we move from Duffins Creek (red squares) to Carruthers Creek (grey diamonds).

Toronto and Region Conservation Authority has also been working with Environment and Climate Change Canada to understand current and nutrient patterns. On the same date as the total phosphorus data above, we have modelled current data in the Western Durham region from Environment and Climate Change Canada (see Figure 9). The white arrows are the current direction and the colours indicate current velocity, increasing from blue to red. On top of the current map is the TRCA total phosphorus data from the same day. This shows that the currents are moving east on this particular day, and that nutrient concentrations decrease along the flow paths of the currents.

Lake Ontario Waterfront nearshore monitoring currents
Figure 9: Modelled current map for June 17, 2008 (current results obtained through Ontario Power Generation project; courtesy of Environment Canada) with TRCA total phosphorus concentrations overlain.

Take Home Messages

  1. Alongshore transport is important!
  2. Alongshore currents are much faster than onshore-offshore currents.
  3. Tracking currents can explain some of the nutrient distributions we see in the nearshore.