Methodology

Sediment movement and balance are key drivers of river health, habitat, and channel dynamics. The sediment regime assessment evaluates three metrics: watershed sediment supply, local sediment inputs, and sediment continuity/transport. Each metric was graded to assess habitat function and sediment conditions. Lower scores often indicate the presence of a dam or diversion structure within the reach.

SEDIMENT METRIC WEIGHTING

WATERSHED SUPPLY: LAND DEVELOPMENT INDEX

The amount and type of sediment delivered from the surrounding landscape into a river system is influenced by factors like geology, land use, vegetation cover, and climate. A well-balanced sediment supply supports the natural formation of bars, banks, and floodplains, while too much or too little sediment can lead to harmful buildup or erosion that destabilizes the channel and disrupts habitat. The Land Development Index (LDI), derived from the National Land Cover Dataset, is used to grade this metric as a reliable indicator of human-driven impacts on sediment supply within the watershed.

WATERSHED SUPPLY: WILDFIRE BURNED AREAS

Wildfires significantly alter sediment dynamics by removing vegetation, changing soil properties which can dramatically increase the amount of sediment delivered to streams. Elevated sediment loads can degrade fish habitat, reduce water quality, and disrupt natural sediment regimes. Stream reaches are graded based on the percentage of their contributing watershed affected by wildfire, with higher burned percentages indicating a greater risk of excess sediment supply and potential need for remediation.

LOCAL SUPPLY

Local sediment supply refers to sediment contributions from nearby sources such as bank erosion and channel incision, which help maintain habitat complexity and natural channel form. When local sediment sources are disrupted by dams, diversions, or other barriers, rivers can become sediment-starved, leading to erosion, channel instability, and declines in fish and macroinvertebrate communities. Stream reaches were graded using a desktop analysis of aerial imagery to identify the percentage of reach length showing signs of impairment such as sediment deposition or lateral bank migration.

CONTINUITY AND TRANSPORT: DAMS

Dams intercept the natural movement of sediment through a river system, disrupting the balance of sediment continuity and transport that healthy streams depend on. For this assessment, the percentage of each reach's tributary drainage area impacted by dams is calculated and used as the basis for grading, with smaller impacted areas receiving higher scores. This metric captures the cumulative influence of multiple upstream dams where applicable.

CONTINUITY AND TRANSPORT: DIVERSION STRUCTURES

Diversion structures redirect flow, reducing the natural flow regime of a river which disrupts sediment continuity and transport. To assess this impact, the number of diversion structures within each reach's drainage area is counted and divided by the drainage area to produce a density value representing structures per square mile. Reaches with higher diversion densities receive lower grades, reflecting the greater disruption to natural sediment passage.