6.4 Lake and reservoir hydrology

Lake and Reservoir Hydrology

Lakes and reservoirs store massive volumes of freshwater, and their behavior depends on the balance between water coming in and water going out. Understanding this balance, along with how water moves and mixes within a lake, is central to managing water supply, predicting water quality, and protecting aquatic ecosystems.

Components of Lake Water Balance

The water balance of a lake is straightforward in concept: if more water enters than leaves, the lake level rises; if more leaves than enters, it drops. The challenge is that several inflow and outflow pathways operate simultaneously.

Inflows bring water into the lake from three main sources:

• Surface water inflows via rivers and streams are typically the largest input. The Amazon River, for example, delivers enormous volumes to its downstream floodplain lakes.
• Groundwater inflows occur where the water table sits higher than the lake surface, allowing water to seep in through the lakebed. Lakes connected to major aquifer systems (like the Floridan Aquifer) can receive substantial groundwater contributions.
• Direct precipitation as rain or snow falling onto the lake surface. For very large lakes like the Great Lakes, this can be a significant fraction of total inflow simply because of the huge surface area.

Outflows remove water through several pathways:

• Surface water outflows through outlet rivers or streams. The Niagara River, for instance, drains Lake Erie into Lake Ontario.
• Groundwater outflows as water seeps from the lake into surrounding aquifers. This happens when the lake surface sits higher than the local water table.
• Evaporation from the lake surface, driven by solar radiation, wind, temperature, and humidity. In arid regions, evaporation can dominate the water balance. The Aral Sea's dramatic shrinkage is partly due to high evaporation combined with reduced inflows.
• Human withdrawals for drinking water, irrigation, or industrial use. Lake Mead, which supplies water to millions in the southwestern U.S., has seen significant level declines partly from sustained withdrawals.

Storage changes reflect the net balance over time:

• Water level fluctuations occur as the lake adjusts to shifting inflows and outflows. Lake Chad has shrunk by roughly 90% since the 1960s due to a combination of climate change and increased water use.
• Seasonal variations are common. Many lakes rise during wet seasons and fall during dry seasons as precipitation, snowmelt, evaporation, and withdrawal patterns shift throughout the year.

Residence Time and Flushing Rate

Residence time is the average length of time a parcel of water stays in a lake before leaving. It tells you how quickly (or slowly) the lake's water gets refreshed.

$T_r = \frac{V}{Q_{out}}$

where $T_r$ is residence time, $V$ is lake volume, and $Q_{out}$ is the total outflow rate.

A large, deep lake with relatively small outflows will have a long residence time. Lake Superior's residence time is about 191 years, while Lake Baikal's exceeds 300 years. A smaller lake with high throughflow might have a residence time of just weeks or months.

Flushing rate is simply the inverse of residence time:

$F = \frac{1}{T_r} = \frac{Q_{out}}{V}$

It represents how many times per unit time the lake's entire volume is theoretically replaced. Why does this matter? Lakes with short residence times (high flushing rates) can recover more quickly from pollution because contaminated water is replaced faster. Lake Erie, with a residence time of only about 2.6 years, responded relatively quickly to phosphorus reduction efforts in the 1970s and 80s. A lake with a residence time of centuries would take far longer to flush out the same pollutant.

Thermal Stratification in Lakes

During warmer months, many lakes develop distinct horizontal layers based on temperature differences. Since water density depends on temperature (freshwater is densest near 4°C), these temperature layers resist mixing and effectively divide the lake into separate compartments.

The three layers, from top to bottom:

• Epilimnion: The warm, well-mixed upper layer. Wind and wave action keep this layer turbulent and relatively uniform in temperature. This is where most photosynthesis occurs.
• Metalimnion (thermocline): The transition zone where temperature drops rapidly with depth. This steep temperature gradient acts as a barrier to vertical mixing between the upper and lower layers.
• Hypolimnion: The cold, dense bottom layer with very limited mixing. Oxygen in this layer is not replenished from the surface during stratification, which can lead to low-oxygen (hypoxic) conditions, especially in nutrient-rich lakes.

Seasonal mixing cycles break down this stratification:

1. In summer, strong solar heating creates stable stratification with a warm epilimnion over a cold hypolimnion.
2. In fall, the surface cools until the temperature difference between layers disappears. Wind can then mix the entire water column. This is fall turnover.
3. In winter, some lakes develop inverse stratification, with colder (but less dense, since water below 4°C becomes less dense) water near the surface and slightly warmer water below. Ice may form on top.
4. In spring, ice melts and surface water warms back toward 4°C, again eliminating the density difference. Wind drives spring turnover, mixing the full water column once more.

These turnover events are critical because they redistribute dissolved oxygen downward and bring nutrients from the bottom sediments up to the surface, fueling biological productivity.

Two key mixing mechanisms drive this process:

• Wind-driven mixing generates turbulence that stirs the upper layers and, during turnover, can mix the entire lake.
• Convective mixing occurs when surface water cools, becomes denser than the water below it, and sinks. This vertical circulation is what initiates turnover in fall and spring.

Human Impacts on Lake Hydrology

Eutrophication is one of the most widespread threats to lake health. It occurs when excess nutrients, primarily nitrogen and phosphorus, enter a lake and fuel explosive algal growth.

Common nutrient sources include agricultural runoff carrying fertilizers, municipal wastewater discharge, and stormwater from developed areas. The consequences follow a predictable chain:

1. Nutrient loading increases.
2. Algal blooms develop, sometimes producing toxins harmful to humans and animals.
3. When the algae die, decomposition by bacteria consumes dissolved oxygen.
4. Oxygen depletion (hypoxia) in deeper water kills fish and other aquatic organisms.

Lake Erie experienced severe eutrophication in the mid-20th century and, despite improvements, still suffers harmful algal blooms driven by phosphorus from agricultural runoff.

Water level management through dams and diversions reshapes lake and reservoir hydrology. Dams create reservoirs like Lake Nasser (behind the Aswan High Dam) and Lake Powell (behind Glen Canyon Dam), which store water for human use but alter natural flow regimes downstream. Excessive withdrawals can cause dramatic declines. The Aral Sea lost most of its volume after rivers feeding it were diverted for irrigation, collapsing its fisheries and leaving toxic dust on the exposed lakebed.

Invasive species can fundamentally change lake ecosystems. Zebra mussels in the Great Lakes filter enormous volumes of water, increasing clarity but redirecting energy away from open-water food webs. Nile perch introduced to Lake Victoria drove hundreds of native cichlid species toward extinction.

Climate change affects lakes through multiple pathways: altered precipitation patterns change inflow volumes, warmer air temperatures increase evaporation and strengthen or prolong thermal stratification, and shifting ice cover duration affects mixing cycles and habitat. In the Great Lakes, ice cover has declined significantly over recent decades, with cascading effects on evaporation rates and lake levels.