Watershed Flow Regimes and Sediment Transport

Watershed Flow Regimes

• Watershed flow regimes concern how precipitation becomes runoff.
• Hydrographs are visual representations of discharge over time.
• Discharge is the measure of water flowing past a fixed point on a stream.
• Annual precipitation patterns affect watershed response.
  • The Pacific Northwest (PNW) experiences dry summers due to high pressure off the coast.
  • Winter months show geographic disparities in precipitation, with more on the west side of the Cascades.
• The water year starts on October 1, especially relevant for irrigators.
• Snow-dominated streams peak in discharge during late spring/early summer, while rain-dominated watersheds peak in late fall/early winter.
• The form of precipitation (rain or snow) significantly impacts runoff timing.

Rain-Dominated Watersheds

• Rain-dominated watersheds receive most precipitation as rain.
  • These are typically located close to the coast at low elevations.

Snow-Dominated Watersheds

• Snow-dominated watersheds receive most precipitation as snow.
  • They are located at higher elevations and in more continental regions.
  • A significant portion of regional water resources is stored as snow during winter, melting in spring.
• Snow Water Equivalent (SWE) is the amount of liquid water in snowpack.
  • Denser snow has a higher SWE.
  • Snow acts as a storage mechanism, delaying runoff until melting occurs.

Rain and Snow-Dominated Watersheds

• Rain/snow-dominated watersheds (transient snow) are a mix.
  • Lower portions receive rain, while higher portions receive temporary snow.
  • They lie between continental, high-elevation, and low-elevation coastal watersheds.
• During storms, snow-dominated watersheds show less discharge initially, as precipitation is stored as snow.
• As temperatures increase in spring, snow-dominated watersheds experience increased discharge from snowmelt.
• Rain-dominated watersheds respond quickly to rainfall events.
• Rain/snow-dominated watersheds exhibit a hybrid pattern.

Nooksack Watershed Example

• The Nooksack Watershed demonstrates these patterns in action.
• The North Fork is primarily snow-dominated, while the South Fork is primarily rain-dominated.
• The gap between discharge in the North and South Forks reflects water stored as snow in the North Fork basin.
• Snowpack is critical for summer water supply for agriculture and maintaining in-stream temperatures for fish.
  • Whatcom County's agriculture depends on snowmelt due to a mismatch between precipitation and water demand.

Climate Change Impacts

• Under a warming climate, more watersheds are expected to become rain-dominated.
• The PNW is projected to become wetter in the winter but warmer, leading to more rain and less snow.
• This results in "shifting the hydrograph."
  • The historical streamflow peaks in May/June due to snowmelt.
  • Warming shifts the peak earlier, resembling a rain-dominated pattern, with reduced water resources in summer.
• The greatest reduction in summer streamflow occurs in high-elevation watersheds.
• The loss of natural storage capacity in snowpack poses a major threat.
• Possible solutions include building new dams to replace lost storage capacity.
• Climate change may lead to increased water stress for vegetation and increased irrigation demands.

Sediment Transport

• Sediment transport involves the movement of weathered rock by flowing water.
• The stream network is a circulatory system for the Earth's surface.
  • It carries sediments downstream, depositing or picking them up along the way.
• Particle sizes larger than dissolved minerals are considered.

Types of Sediment Transport

• Dissolved Load: Dissolved salts and minerals are carried in the water.
• Bed Load: Sediments that roll, bounce, or drag along the bottom of the stream.
• Suspended Load: Sediments that are floating in the water column.
  • A muddy river indicates a high suspended load.
• Discharge is related to the size of particles a stream can move and when sediments deposit.
  • discharge = depth \times width \times velocity

Water Velocity and Particle Size

• Water velocity directly affects sediment erosion and transportation; as water velocity increases, larger particles can be moved.
• Faster-moving water mobilizes larger particle sizes.
• During flood events, large boulders can be moved. When the water slows, larger particles drop out first. Sediments are sorted by size.
• Sediment moved by water tends to be sorted.

Dams and Sediment Transport

• Dams disrupt natural sediment transport.
• Dams create artificial base levels, where the water velocity slows, and sediments drop out, acting as sediment traps.
• Reservoirs behind dams accumulate sediment, reducing water storage capacity and causing problems downstream."

Alluvium and Colluvium

• Alluvium: Sediments rounded and smoothed by tumbling action in a river or stream, typically well-sorted and stratified.
• Colluvium: Material deposited by gravity (e.g., glaciers, landslides), unsorted and angular.

Dams as Artificial Base Levels

• Every stream has a natural base level, where it meets non-moving water like the ocean.
• Deltas, fan-shaped accumulations of sediment, form where river water slows and deposits sediments.

Elwha River Dam Removal

• Two dams on the Elwha River were removed to open salmon habitat.

Sediment Accumulation in Reservoirs

• Sediment accumulates in reservoirs because the velocity of inflowing water slows, causing deposition. This process builds a delta within the reservoir.
• The delta surface slopes toward the dam.

Post-Dam Removal Observations

• After dam removal, the river incised into the accumulated sediment, reestablishing a mainstream channel.
• This incision led to sediment erosion and downstream transport.
• Terraces or shelves of sediment sit above the new stream channel.
• Initial sediment load increase was detrimental to fish, but long-term success with salmon reintroduction occurred.
• Estuary and nearshore environment benefited from sediment redistribution.

Long-Term Impacts

• Dam removal can rebuild deltas. A land surface can be built up.
• Sediment is important for fish to build reds. There is a channel complexity.
• Free flowing channels move sediment, a process that can be restored through dam removal.