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.