The level of a lake reflects a balance between water coming into the lake (from rainfall, rivers and groundwater) and leaving it (from evaporation, seepage and outflow). Rainfall, ultimately the source of all water entering a lake, is usually highest in winter months. If a lake has no outlet, its level will rise and fall according to the rainfall. But with an outlet the level will stay stable, although the outflow will vary.
Levels at some major lakes have been recorded on some of the major lakes for up to a century. These are some of the most reliable water measurements in New Zealand. Today lake levels are recorded automatically at many sites, and this data is immediately radioed to central recording sites.
Seiches: oscillating water
Steady winds exert drag, which raises lake levels on the downwind side, and gravity depresses them in the opposite direction. In larger lakes this causes lake-wide fluctuations known as seiches. Levels on the shoreline gradually rise and fall by up to a metre. Each lake has a distinctive seiche period between high-water levels, ranging between 20 and 55 minutes. For example:
- Taupō: 36 minutes
- Wānaka: 39 minutes
- Wakatipu: 52 minutes.
The lake environment
The apparent stillness of a lake conceals a complex environment. Although the water movement in a lake is weak compared to a river or the sea, it is of crucial importance for water quality as well as plant and animal life.
Water in a lake is always moving slowly under the influence of wind and the earth’s rotation. Small differences in water density can also cause water movement because denser water sinks below less dense water. Cold water is denser than warm water, and sediment-laden water from a flooded river is denser than clear water.
In contrast to rivers, which are constantly replenished by fresh water, lake waters are held in a basin, often for years or tens of years. Because the lake water is only slowly renewed, lakes are vulnerable to the effects of human activity, especially changes in the area that drains into the lake. Lake ecology and water quality are easily damaged, and may take longer to improve than in a river.
Long lost lake
Parts of Central Otago lie on lake silts. Geologists believe these formed in a large lake (named Lake Manuherikia), which existed 16–18 million years ago. Fossils show that the lake was home to water birds, fish, crocodiles and surrounding eucalyptus trees, and that the climate was much warmer then.
Water layers: spreading oxygen
For much of the year the waters of all but the smallest lakes have a distinct upper layer (epilimnion) and a lower layer (hypolimnion).
During spring and summer the lake surface is warmed by the sun. Wind movement keeps the epilimnion mixed, aerated and at a uniform temperature. This upper layer floats on the cooler, denser water of the hypolimnion. The two layers are separated by a narrow zone called the thermocline.
During autumn and winter the epilimnion cools and becomes denser. When it reaches the same temperature as the hypolimnion the water overturns and mixes. This annual event is critical for the life of the lake because it allows oxygen to be spread throughout the water. This affects all the plants and animals living there.
A giant’s grave
In Māori tradition, Lake Wakatipu is the grave of the giant Matau, who abducted a beautiful high-born woman, Manata. She had a suitor named Matakauri, who rescued her and returned to kill the giant, setting fire to the dried fern around him. Matau was suffocated by the smoke and, with flames licking at his side, he drew up his legs. His burning body, with raised knees, formed a distinctive curving chasm that filled with water. His heart continues to beat, causing the regular changes in lake level.
Sediment in lakes
Lakes act as a trap for sediment. Most of the sediment that is carried by rivers into lakes, or forms at the bottom when floating organisms (plankton) die, stays in the lake basin. Over time, a lake becomes shallower and eventually fills in. Most New Zealand lakes are young in geological terms – generally less than 10,000 years old.
Some glacial lakes have been repeatedly dug out as glaciers advanced, then gradually filled in with sediment during the interglacial period.
The annual rate of sedimentation in New Zealand lakes varies between 1 and 200 millimetres. The highest rates are in reservoirs (artificial lakes). These are on rivers that flow through highly eroded country, and carry a high sediment load.