Lac La Biche

Alberta's lakes provide desirable natural places, recreational opportunities and valuable habitat for aquatic and terrestrial life. Lakes are highly valued so those who use them and live nearby will notice their fluctuations and changes.

Alberta has a tremendous number and variety of lakes. These range from small, landlocked lakes that are shallow, saline and experience wide fluctuations in water levels - sometimes even drying up for a period of time - to very deep lakes that cover hundreds of square kilometres, and have relatively stable water levels.

Lakes store water. They can form wherever water can accumulate in a low spot relative to the surrounding countryside, provided that the amount of water coming into the depression is larger than the water that is escaping. Once a lake is formed, it will continue to expand or contract in size until the water entering the lake (Inflow) equals the amount leaving the lake (Outflow). This relation or balance between inflows and outflows is the basis of the "Water Balance Equation", used by hydrologists to assess the behaviour of lakes:

    Inflows are made of

• Precipitation - rain and snow that falls directly onto the lake;
• Surface Runoff - runoff from the lake's catchment area that flows into the lake through defined streams and channels or through the upper soil layers;
• Groundwater Inflow - groundwater entering the lake via buried channels and connections to aquifers;
• Man-made Diversions - that bring water from outside the lake catchment area into the lake.

    Outflows are made of

• Evaporation - water evaporated from the lake surface area;
• Surface Outflows - if the lake has an outlet channel and the lake level is high enough for it to spill downstream;
• Groundwater Outflow - lake water leaving the lake through groundwater sources;
• Man-made diversions - human diversions of lake water for industrial, agricultural or domestic uses.

    Change in Storage is calculated by

• Using bathymetric surveys that relate lake level to lake volume.

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In general, the largest components of the water balance are precipitation, evaporation and surface runoff. As these parameters are the result of climatic conditions, wet or dry weather conditions can create a short-term imbalance between the inflows and outflows causing the lake to rise temporarily when inflows exceed outflows and the lake to temporarily decline when inflows are less than outflows. These deviations from normal lake levels may persists for days, weeks, months, or years, until inflow and outflow return its normal balanced conditions. An example of these short-term fluctuations is the temporary rise in lake levels experienced in spring from snowmelt runoff and the temporary declines experienced in the early fall when precipitation is low and the lake continues to lose water through evaporation and outflow.

In the case of man made diversions and/or climate change, the balance between inflows and outflow may be permanently altered to some new level. This new balance will be at a higher water levels if the human diversions bring in additional inflows or if climate change results in higher amounts of precipitation or lower amounts of evaporation. The new balance will be at lower lake levels if human diversions remove water from the lake or if climate change brings lower quantities of precipitation or higher amounts of evaporation. As the precipitation falling on the lake generally has much lower levels of nutrients and minerals than the water which is stored in the lake, and as evaporation removes water from the lake while leaving nutrients and minerals behind, wet and dry weather cycles can also impact the water quality of a lake.

With the exception of high mountain areas, most parts of Alberta have higher mean annual evaporation than precipitation. Therefore, if lakes were to rely solely on the rain and snow falling directly on the lake surface they would dry up over time as evaporation losses consumed ever greater portions of the stored water. Lakes offset this built in "water deficit" by collecting surface water running off the surrounding terrain. The surrounding land area whose surface runoff drains into the lake, or streams flowing into the lake, is called the catchment area or watershed. The relative size of the catchment is important, as measured against the surface area of the lake, in that it determines the stability (fluctuations) of the lake over time. In Alberta, a lake needs a catchment area that is about three to four times the surface area of the lake to be sustainable over the long-term. This ratio will generally be smaller for mountain areas that has higher precipitation than the provincial average and will be greater for the south-eastern portion of the province which have higher evaporation and lower precipitation than the provincial average.

In general, lakes with larger relative catchment sizes tend to:

• fluctuate at levels at or above the outlet elevation;
• have reduced outflow, rather than drastically reduced levels (below the outlet) during prolonged dry spells, and will take longer to drop below their outlet level; and
• respond more quickly when conditions become wetter.

Lakes with smaller catchments tend to:

• fluctuate around or below their outlets;
• stay below their outlet (no outflow) during longer dry periods and only spilling when it has been wet enough over time to build up the lake levels again; and
• take longer to recover from a dry period once a wetter cycle does occur.