In lakes, the main heat transfers occur at the water surface, which exchanges heat with the air and receives heat from solar radiation. The lake surface temperature therefore follows approximately air temperature through the year, although it varies much slower. The deepwater temperature of deep lakes is determined by mixing processes and in deep lakes remains cold throughout the year.
Typical thermal regime for a low-elevation lake in Switzerland (see plot on the right):
In winter, the lake temperature is rather homogeneous from the bottom to the surface of the lake, usually at 4-6 °C. In spring, the lake surface starts to warm up. As warmer water is less dense, it floats on top of the deeper, colder layers. These deeper layers are then isolated and warm up only slowly, for example when strong wind provokes mixing. In summer, there is usually a warm water layer (>15 °C) down to 10-25 m depth. Below it, the temperature decreases rapidly and, below 30 m, water remains cold (<8°C). In fall, the lake surface cools down, creating mixing with the deeper layers. As a result, a large layer (20-40 m deep) of ~10 °C water can often be observed at the end of the fall. Wind influences much the mixing: strong storms can push this water down to larger depths and contribute to homogenize the lake temperature. If, over the winter, temperature decreases enough so that it is about the same in the whole lake, vertical mixing can occur, whereby water is exchanged through the whole water column, renewing oxygen in the deeper layers. This is essential for the lake ecosystem.
The temperature regime of a lake can be strongly altered if this lake is used as a reservoir for hydroelectricity production - especially if water is withdrawn from the bottom of the lake.
The average seasonal temperature profiles of specific Swiss lakes can be visualized here.
Rivers are mainly fed by rain and by the melting of snow and ice. Lowland rivers are usually primarily fed by rain and have a pluvial flow regime. The flowrate responds quickly to precipitation and is therefore high during rainy periods and low during dry periods (typically in the fall). The feeding of the river by groundwater acts as a buffer, as groundwater can store rain for much longer than rivers. Highland rivers are often primarily fed by the melting of snow and ice. The flowrate is then low in winter and increases during spring and summer. The maximum flow rate occurs at the period of the maximum snowmelt in the drainage basin (typically in the spring).
In Switzerland, most rivers have part of all of their source in mountainous regions. Therefore, snowmelt is a key driver of the flowrate. A map of the flow regime of most Swiss rivers can be found here.
The flowrate in rivers downstream of lakes is typically more constant and often artificially regulated. Below hydropower reservoirs, it is modified by the hydropower discharge which generally shift part of the discharge from summer to winter.
Close to its source, a river has a temperature mainly determined by this source, which can for example be a mountainous drainage basin, a lake or groundwater. The further from the source, the more the river temperature becomes influenced by air temperature and other drivers.
In Switzerland, alpine rivers upstream of large lakes are typically not suited for heat extraction in winter (temperature < 3°C) but well-suited for heat in summer (temperature < 15°C). On the contrary, lowland rivers downstream of large lakes are typically well-suited for heat extraction in winter (temperature > 4°C) but may periodically become too warm for heat disposal in summer. Rivers with a groundwater source can be suited for both uses.