Changes in hydrology

Key takeaways
 

• The Northeast U.S. (NEUS) hydrologic cycle is impacted by climate change.
• Intensification of the hydrologic cycle due to climate change can increase the delivery of nutrients and pollutants to downstream and coastal habitats.
• The annual average river and stream flows increased in late 20th century across the NEUS, while peak flows have stayed more constant.
• The NEUS is experiencing rising streamflow and baseflow in the winter months but a decline in summer/fall.
  Snow droughts adversely impact streamflow.
• Earlier snow melt and ice break up is altering seasonal water level patterns (hydroperiod) in water bodies and wetlands in the NEUS.
• Changes in spring snowmelt timing and magnitude impact aquatic species that depend on seasonal flows for life cycle event.
• Groundwater regulates stream temperature and flow, providing coldwater refugia for aquatic organisms, but winter warming and earlier spring melting are reducing groundwater reserves.

Climate-induced changes in precipitation, winter conditions, and extreme storm events have increased base and average stream and river flows in many parts of NEUS. Land use practices, water withdrawals for human use, and development are also influencing hydrological conditions of water bodies and aquifers. Intensification of the hydrologic cycle (evaporation, condensation, precipitation, etc.) due to climate change and extreme precipitation events can increase the delivery of nutrients and pollutants to downstream and coastal habitats. This has important implications for food-web structure and ecosystem function, such as making poor water quality events (e.g., excessive nutrient loading) and the incidence of waterborne disease more likely. Hydrological conditions and the ability of aquatic species to travel from stream to stream (i.e., aquatic connectivity) are important for aquatic fish and wildlife survival. Human structures that intersect with aquatic systems, such as dams, culverts, and road crossings, are potential barriers to fish and wildlife movement.

Stream and River Flows

Climate change directly impacts stream and river flows through changes in precipitation, temperature, and evapotranspiration (the sum of evaporation and the movement and exchange of water among plants, land, waterbodies, and the atmosphere). However, impacts will vary among basins based on:
•    The types and amount of vegetation and forest cover along the banks of streams and rivers
•    Extent of nearby urbanization
•    Underlying geology
•    Elevation
•    Basin slope
•    Groundwater contributions

Annual average river and streamflow rates have increased during the last part of the 20th century throughout the NEUS, even though peak flow rates have remained generally constant. Streamflow records have indicated rising streamflow and baseflow in the winter months but a decline in summer/fall, suggesting changes in timing but not necessarily in magnitude (Ficklin et al 2016). Flows are changing their timing more strongly than perhaps the overall magnitude. Minimum daily streamflow appears to be more strongly correlated to summer precipitation (Crossett et al 2023). Recent studies indicate that snow drought has a bigger impact streamflow than previously thought (Hammond 2024).“Low flows” play a crucial role in maintaining aquatic ecosystems as they provide a base level of flow (i.e., sustained flows composed of normal inputs from groundwater and precipitation) and help control stream water temperatures, particularly during summer. What constitutes a low or minimum flow varies by study, location, and season, but is generally the lowest flow for that site (excluding zero level flows or no water) over a defined time period (e.g., a week).

Seasonal and Decadal Influences on Flows

Earlier winter-spring peak stream and river flows (in the range of 7-10 days) have been observed in the NEUS and are thought to be linked to earlier snowmelt and increased rain-on-snow episodes. This trend is projected to continue during the 21st century. A shift toward higher winter flows and lower spring and summer flows has been documented in the Connecticut River Watershed using climate-driven streamflow simulations. Changes in the timing and the magnitude of spring snowmelt in the eastern U.S. are crucial to maintain ecosystem functions since some aquatic species rely on seasonal streamflows for critical life cycle events. Larger peak flows can contribute to increases in river scour magnitude and frequency and negatively affect the survival of some species and life stages. In addition, changes in winter climatic conditions due to decreased snowpack, increased temperatures, and precipitation may increase nutrient and pollution inputs into regional watersheds, negatively impacting water quality and downstream habitats.
Large shifts in the magnitude of maximum annual streamflows have also been documented in many stream and river basins in New England. These shifts are associated with decadal climatic cycles such as the North Atlantic Oscillation, a natural, somewhat random phenomenon that produces changes in atmospheric pressure and sea level heights. These shifts are abrupt and short-lived, lasting no longer than a year. Larger peak seasonal flows and velocities contribute to increases in river scour magnitude and frequency, and affect egg burial depths, displacement, and survival of some fish and animals, such as freshwater mussels.

Groundwater Influences on Streams and Rivers

Groundwater inputs into streams and rivers are also important regulators of temperature and flow. Groundwater can contribute to coldwater refugia for fish and other cold-adapted aquatic organisms, especially during summer and low flow periods. Changes due to winter warming and the timing of spring melting are expected to reduce groundwater reserves during subsequent seasons. In addition, drought conditions affect water supply to freshwater systems and impact their chemistry and temperature; the magnitude of these effects is partially dependent on whether the system is primarily fed by groundwater or surface waters (e.g., streamflows and rainfall). Alternatively, recent studies in New England suggest that groundwater table levels are rising, potentially increasing the risk for flooding.

Lakes and Pond Levels

Lakes are indicators of climate change in part because they reflect climatic warming and are directly influenced by changes to the surrounding terrestrial landscape. Lakes are warming at rates similar to air temperatures on a global scale. Responses of lakes to climate change are complex and variable, and often depend on individual basin characteristics, such as:
 

• Water depth
• Surface water influences
• Groundwater influences
• The local landscape

Nearby vegetation and forest coverage will affect how exposed or sheltered small and large water bodies are to wind, which influences the vertical distribution of water temperatures (i.e., temperatures at varying depths). Vegetation can also provide protection from erosion and the influx of nutrients and pollutants to lakes and ponds.

Climate Induced Changes in Nutrient Levels to Lakes and Ponds

Increased precipitation and extreme weather events due to climate change are increasing the delivery of nutrients to lakes from terrestrial runoff, which results in degraded water clarity and increased stratification (i.e., increasing the difference between warmer surface waters and colder bottom waters). These chemical and physical changes have important impacts on photosynthesis and aquatic food-webs. Lakes and ponds are also impacted by atmospheric deposition of sulfur and nitrogen oxides, which change the chemistry of waters through acidification.
Acidification (or deacidification) affects the levels of dissolved organic carbon (DOC) as well as water temperature and clarity. DOC is generated from the decomposition of living matter, such as vegetation and animals, and represents the primary form of carbon transported and exchanged within soils and aquatic ecosystems. DOC is naturally higher in certain types of waterbodies, such as those with surrounding wetlands or peatlands, but can also be the result of pollution. Too much DOC in a system can result from high nutrient loading that overstimulates primary production, for example from an algal bloom. This leads to increased rates of decomposition and ultimately hypoxic (low oxygen) conditions that can be deadly to other organisms.

Seasonal Changes in Ice and Water Levels

Warmer winter air temperatures are impacting ice cover and duration over lakes and ponds in NEUS. Ice on lakes and ponds has been breaking up earlier by a week or more in New England. Earlier melting of snow and ice during winter and spring are altering the seasonal patterns of water levels, or the hydroperiod, in water bodies and surrounding wetlands. Future projections of climate change are expected to reduce the hydroperiod of ephemeral ponds and wetlands (e.g., vernal pools) due to temperature effects on evaporation rates and changes in seasonal precipitation rates.