U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2015–5033

Prepared in cooperation with The Nature Conservancy, Washington State Department of 

Ecology, and U.S. Environmental Protection Agency

Geospatial Assessment of Ecological Functions and 

Flood-Related Risks on Floodplains along Major Rivers in the 

Puget Sound Basin, Washington

Geospatial Assessment of Ecological 

Functions and Flood-Related Risks on 

Floodplains along Major Rivers in the 

Puget Sound Basin, Washington

By Christopher P. Konrad

Prepared in cooperation with The Nature Conservancy, Washington State 

Department of Ecology, and U.S. Environmental Protection Agency

Scientific Investigations Report 2015–5033

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director 

U.S. Geological Survey, Reston, Virginia: 2015

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Suggested citation:

Konrad, C.P., 2015, Geospatial assessment of ecological functions and flood-related risks on floodplains along major 

rivers in the Puget Sound Basin, Washington: U.S. Geological Survey Scientific Investigations Report 2015–5033, 

28 p., http://dx.doi.org/10.3133/sir20155033.

ISSN 2328-0328 (online)
iii

Contents

Abstract...........................................................................................................................................................1

Introduction ....................................................................................................................................................1

Purpose and Scope ..............................................................................................................................1

Description of Study Area ............................................................................................................................2

Overview of Assessment..............................................................................................................................2

Source Data...........................................................................................................................................4

Derived Datasets...................................................................................................................................4

Land Forms—Valley Bottoms, High Floodplains, Low Floodplains, and River Areas.......4

Connectivity ..................................................................................................................................8

Fine-Resolution Assessment........................................................................................................................8

Floodplain Functions...........................................................................................................................12

Function 1. Store and Convey Floods .....................................................................................12

Function 1a. Store Floodwater.................................................................................................13

Function 1b. Convey Floods......................................................................................................13

Function 2. Regulate Sediment and Wood Supplies in River Networks ..........................13

Function 2a. Supply Sediment .................................................................................................14

Function 2b. Retain Sediment and Particulate Organic Material ......................................14

Function 3. Retain and Transform Nutrients and Contaminants .......................................14

Function 3a. Denitrification ......................................................................................................14

Function 3b. Retain and Transform Contaminants................................................................15

Function 4. Support Forest Ecosystems.................................................................................15

Function 5. Provide Aquatic Habitats .....................................................................................15

Flood-Related Hazards.......................................................................................................................15

Hazard 1. Inundation .................................................................................................................15

Hazard 2. Channel Occupation ................................................................................................16

Hazard 2a. Bank Erosion...........................................................................................................16

Hazard 2b. Avulsion ...................................................................................................................17

Flood-Related Exposures...................................................................................................................17

Exposure 1. Key Facilities.........................................................................................................17

Exposure 2. Land Uses..............................................................................................................17

Exposure 3. Roads......................................................................................................................17

Coarse-Resolution Assessment ................................................................................................................17

Floodplain Functions...........................................................................................................................20

Function 1. Store and Convey Floods .....................................................................................20

Function 2. Regulate Sediment and Wood Supplies in River Networks ...........................20

Function 3. Retain and Transform Nutrients and Contaminants ........................................20

Function 4. Support Forest Ecosystems.................................................................................20

Function 5. Provide Aquatic Habitats .....................................................................................20

Flood-Related Hazards.......................................................................................................................21

Flood-Related Exposure.....................................................................................................................21

Overview of Assessment Outputs.............................................................................................................21

Summary........................................................................................................................................................25

Acknowledgments.......................................................................................................................................25

References Cited..........................................................................................................................................25
iv

Figures

1. Map showing major river networks and valley bottom areas in the Puget Sound

Basin, Washington........................................................................................................................3

2. Graph showing change in stage from median daily stage for the 50th and 90th

percentiles of annual maximum stage as a function of drainage area at 40 U.S. 

Geological Survey streamgaging stations on unregulated rivers without levees 

with at least 10 years of daily stage records, Puget Sound Basin, Washington ...............7

3. Pie diagram showing connectivity and development of floodplains in Puget Sound 

Basin, Washington......................................................................................................................22

4. Graph showing width of connected floodplain area as a function of drainage area

for mainstem rivers in the Puget Sound Basin, Washington...............................................23

5. Graph showing length of confined and unconfined mainstem rivers in the Puget

Sound Basin, Washington ........................................................................................................24

6. Graph showing cumulative floodplain area for fragment sizes, Puget Sound Basin,

Washington .................................................................................................................................24

Tables

1. Source datasets used in the geospatial assessment of ecological functions and

flood-related risks on floodplains along major rivers, Puget Sound Basin, 

Washington....................................................................................................................................5

2. Derived datasets used in the geospatial assessment of ecological functions and

flood-related risks on floodplains along major rivers, Puget Sound Basin, 

Washington....................................................................................................................................6

3. Summary of models and outputs used for fine-resolution (10-m) assessment of

floodplain function, hazard, and exposure, Puget Sound Basin, Washington ...................9

4. Attributes of floodplain units used in the geospatial assessment of ecological

functions and flood-related risks on floodplains along major rivers, Puget Sound

Basin, Washington .....................................................................................................................18

5. Indices of floodplain function and risk used in the geospatial assessment of

ecological functions and flood-related risks on floodplains along major rivers,

Puget Sound Basin, Washington..............................................................................................19

6. Summary of floodplain connectivity and land use resulting from the geospatial

assessment of ecological functions and flood-related risks on floodplains along

major rivers, Puget Sound Basin, Washington......................................................................23
v

Conversion Factors

[International System of Units to inch/pound]

Multiply By To obtain

Length

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

hectare (ha) 2.471 acre

square kilometer (km2) 0.3861 square mile (mi2)

Datums

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Elevation, as used in this report, refers to distance above the vertical datum. 

Geospatial Assessment of Ecological Functions and 

Flood-Related Risks on Floodplains along Major Rivers in the 

Puget Sound Basin, Washington

By Christopher P. Konrad

Abstract

Ecological functions and flood-related risks were 

assessed for floodplains along the 17 major rivers flowing 

into Puget Sound Basin, Washington. The assessment 

addresses five ecological functions, five components of 

flood-related risks at two spatial resolutions—fine and 

coarse. The fine-resolution assessment compiled spatial 

attributes of floodplains from existing, publically available 

sources and integrated the attributes into 10-meter rasters 

for each function, hazard, or exposure. The raster values 

generally represent different types of floodplains with regard 

to each function, hazard, or exposure rather than the degree 

of function, hazard, or exposure. The coarse-resolution 

assessment tabulates attributes from the fine-resolution 

assessment for larger floodplain units, which are floodplains 

associated with 0.1 to 21-kilometer long segments of major 

rivers. The coarse-resolution assessment also derives indices 

that can be used to compare function or risk among different 

floodplain units and to develop normative (based on observed 

distributions) standards. The products of the assessment are 

available online as geospatial datasets (Konrad, 2015; http://

dx.doi.org/10.5066/F7DR2SJC).

Introduction 

Floodplains are a vital part of the ecological and social 

landscape in the Puget Sound Basin. Ecologically, floodplains 

support diverse and productive biological communities 

that inhabit riparian forests, wetlands, and river channels. 

Culturally, large river floodplains have been a focus for 

fishing, hunting, human settlement, and agriculture because 

of an abundance of fish and wildlife, fertile soils, level land, 

and access to water. The ecological and cultural significance 

of floodplains compels floodplain managers to improve 

ecological function while reducing flood-related risks to 

people. This assessment presents geospatial information about 

ecological functions and flood-related risks along 17 major 

rivers in the Puget Sound Basin that can be used to facilitate 

integrated management of floodplains for multiple objectives.

Purpose and Scope

Five ecological functions and five components of risk 

to people living on and using floodplains were assessed 

along major rivers in the Puget Sound Basin. The assessment 

integrates spatial information about factors that contribute to 

each function and risk component. The output can be used in 

a geographic information system (GIS) to identify locations 

to protect or recover ecological functions, to reduce risks, and 

to understand the types of actions that likely are required to 

achieve those objectives. 

Floodplains support many ecological functions, which 

range from basic services, such as storing sediment and acting 

as the physical template for riparian and aquatic ecosystems, 

to more integrated processes including population dynamics, 

trophic exchanges, and community succession (Wolman and 

Leopold, 1957; Ward and others, 2002; Naiman and others, 

2010). Although floodplain restoration depends on the suite 

of all ecological functions provided by floodplains (National 

Research Council, 1992), integrated floodplain management 

may focus on improving individual functions depending on 

management priorities. The assessment addresses five basic 

functions of floodplains: 

1. Store and convey floods; 

2. Regulate sediment and wood supplies in river networks; 

3. Retain and transform nutrients and contaminants; 

4. Support forest ecosystems; and 

5. Provide aquatic habitats, particularly for threatened and 

endangered salmonids.

These functions were selected for three related reasons. 

They accommodate a range of ecological objectives for the 

floodplain manager. Individually, each function can provide a 

benefit in settings where comprehensive floodplain restoration 

may not be possible. The assessment of these functions can be 

based on a categorization of a limited number of key factors 

rather than requiring a more sophisticated integration of many 

factors into a quantitative estimate of functionality.
2 Geospatial Assessment of Ecological Functions and Flood-Related Risks, Puget Sound Basin, Washington

Cultural activities on floodplains present flood-related 

risks in the Puget Sound region. Generally, risk can be 

assessed in terms of its constituent components including 

hazard (physical process that causes damage), exposure 

(people, property, or activities that are impacted by the 

hazard), vulnerability (severity of damage when the hazard 

occurs), and resilience (capacity to recover after the hazard 

occurs). This assessment provides information about two 

flood-related hazards and three forms of exposure of people to 

those hazards:

1. Inundation hazards;

2. Channel occupation hazards;

3. Key facility exposure;

4. Land use exposure; and

5. Road exposure.

Vulnerability and resilience of human activities on floodplains 

are outside the scope of the assessment.

Functions and risks are assessed in terms of categories 

representing distinct combinations of factors contributing 

to each function or risk rather than a continuous scale of the 

level of function or risk. The categories can be used to infer 

the broad types of management actions required to improve 

function or reduce risk, such as reconnecting floodplains or 

providing flood protection for agricultural practices.

This assessment is structured on geospatial data that are 

available for major river valleys in the Puget Sound Basin. A 

comprehensive assessment of function and risk would require 

additional information about floodplains and development 

of methods for integrating that information, which are 

outside of the scope of this assessment. Determination 

of floodplain function, flood-related hazards, or human 

exposure to flood-related hazards at specific locations requires 

additional investigation (for example, Rapp and Abbe, 2003; 

Latterell and others, 2006; Konrad and others, 2008; Olson 

and others, 2014; U.S. Army Corps of Engineers, 2014). 

Discrepancies between this and other assessments should be 

expected because of the simplified methods and resolution 

of data used in this assessment. Without verification of the 

assessment results, deference should be given to site-specific 

information or more detailed analyses of functions and risks at 

particular locations.

Description of Study Area

The Puget Sound Basin covers 36,700 km2 in western 

Washington State and the Province of British Columbia that 

drain to Puget Sound and the Strait of Juan de Fuca (fig. 1). 

The basin is bounded by the Cascade Range to the east and 

the Olympic Mountains to the west. The southern boundary 

of the basin is defined by the divide between the Chehalis 

River (outside of Puget Sound Basin) and the Nisqually 

and Deschutes Rivers (inside of Puget Sound Basin). Puget 

Sound is bounded to the west by the Strait of Juan de Fuca at 

Admiralty Inlet. Areas along the northern coast of the Olympic 

Peninsula to Cape Flattery, the San Juan Islands and areas west 

of the Cascade crest from the Skagit River to the international 

border with Canada typically are included in the Puget Sound 

Basin for regional planning (Washington State Department of 

Ecology, 2013a). 

This assessment covers the valley bottom areas of the 

17 largest rivers in the basin—Nooksack, Samish, Skagit, 

Stillaguamish, Snohomish, Cedar, Green-Duwamish, 

Puyallup, Nisqually, Deschutes, Skokomish, Hamma Hamma, 

Duckabush, Dosewallips, Big Quilcene, Dungeness, and 

Elwha. Valley bottoms are defined as areas where the land 

surface is less than 10 m above the elevation of the river 

extrapolated across the valley and includes alluvial deposits 

filling valleys (Washington Department of Natural Resources, 

2010) and Special Flood Hazard Areas (SFHA) with less than 

a 0.01 annual probability of flooding (Federal Emergency 

Management Agency, 2013). The study area extends up the 

networks of these rivers to include the valleys of tributaries 

that drain more than 50 km2. The total length of major rivers 

and tributaries in the assessment is 3,290 km. Floodplains 

associated with small rivers and with streams draining directly 

to Puget Sound and streams in the 17 major river basins that 

have a drainage area less than 50 km2 are not included in 

this assessment.

The assessment area spans both mountain and lowland 

valleys, which have distinct geologic histories in the Puget 

Sound Basin. Mountain valleys in the basin were formed 

by a combination of alpine glaciers and incision by rivers. 

Lowland valleys were carved by continental glaciers and 

subsequently filled by glacial and fluvial sediments. Collins 

and Montgomery (2011) describe a typology for valleys in 

the Puget Sound region that further distinguishes lowland 

valleys filled by glacial or post-glacial (fluvial) sediments and 

implications of these distinct histories for channel dynamics.

Overview of Assessment

This assessment compiles and derives spatial information 

on factors affecting ecological functions and flood-related 

risks to people on floodplains. Floodplain functions and 

risks depend on local conditions, such as topography 

and land cover, but also the larger floodplain system that 

extends laterally across a valley, and the position of the 

floodplain in a basin. To address the influence of factors at 

different scales, the assessment is implemented with two 

resolutions—fine and coarse. The fine-resolution assessment 

integrates physiographic and cultural attributes of 10-m 

cells to assess local conditions affecting function and risk. 
Overview of Assessment 3

tac14-0968_fig01

Skagit R.

Green R.

Puyallup R.

Cedar R.

Deschutes R.

Nisqually R.

Duckabush R.

Samish R.

Dungeness R.

Elwha R.

Hamma

Hamma R.

Nooksack R.

Dosewallips R.

Snohomish R.

Stillaguamish R.

Duwamish R.

Skokomish R.

Big

Quilcene R.

Chehalis R.

San 

Juan

Islands

Strait of Juan de Fuca

Puget Sound

BRITISH COLUMBIA, CANADA

Olympic

Mountains Cascade Range

WASHINGTON STATE

Admiralty

Inlet

Cape

Flattery

PACIFIC

OCEAN

124° 123° 122° 121°

49°

48°

47°

0 10 20 30 40 50 MILES

0 10 20 30 40 50 KILOMETERS

WASHINGTON STATE

Figure

location

Puget Sound basin

Valley bottom

EXPLANATION

Figure 1. Major river networks and valley bottom areas in the Puget Sound Basin, Washington.
4 Geospatial Assessment of Ecological Functions and Flood-Related Risks, Puget Sound Basin, Washington

The coarse‐resolution assessment summarizes the results of 

the fine-resolution assessment for floodplain units (FPU) that 

span laterally across a river valley and longitudinally along a 

major river segment between tributaries or transitions in valley 

gradient or confinement. The coarse-resolution assessment 

also provides metrics for each FPU that indicate the function 

or risk of the larger floodplain system. All datasets were 

processed in ArcGIS 10.0 (Environmental Systems Research 

Institute, 2013) using either grid or shapefile formats. The 

names of datasets referenced in the text are italicized (for 

example, land_z_ned) including interim datasets generated 

and used in processing steps but not included as products with 

this report

The uncertainty in the assessment outputs depends on 

uncertainty in source data but also on uncertainty in how 

multiple factors influence a function or risk and how those 

factors influence scales over a larger floodplain system. 

Verification of highly derived outputs that integrate multiple 

factors over large floodplain systems was outside of the scope 

of this assessment. Instead, each output is based on a limited 

number of factors; those factors generally were represented 

as categories rather than a continuous value to clarify the 

interpretation of results, and spatial scaling is limited to 

area‐based summation or averaging. 

Source Data

The initial step in the assessment was to compile 

20 publically available datasets on topography, geology and 

soils, hydrography, land cover and land use, water quality, and 

use of rivers by salmonids listed as threatened or endangered 

under the Endangered Species Act (table 1). These source 

data were projected to Universal Transverse Mercator Zone 

10 North (UTM Zone 10 N), North American Datum 1983 

(NAD 83) coordinate system and clipped to valley bottom 

areas to reduce the size of the datasets. Source data were 

used to derive land forms and hydrographic characteristics of 

floodplains including their connectivity with rivers. 

The assessment uses both the 1/3 arc-second National 

Elevation Dataset (NED) and higher resolution (1.8 m) 

topography obtained from aerial light detection and ranging 

(lidar) surveys (table 1). NED was used to delineate valley 

bottoms, valley gradients, and high floodplains. Lidar 

data were used to delineate rivers and low floodplains, 

which would not be well resolved in the NED. Lidar data 

were available for 74 percent of major river length and 

95 percent of the assessment area from the Puget Sound Lidar 

Consortium (2011). The lidar supermosaic does not cover the 

western Olympic peninsula, which includes the Big Quilcene, 

Dosewallips, Duckabush, and Hamma Hamma Rivers, as well 

as some high-elevation segments of other rivers.

The resolution of the other geospatial sources is variable. 

Most features (hydrography, roads, levees, railroads, geology, 

soils, and land use) were based on vector data (point, line, 

or polygon) where resolution varies with scale ranging from 

1:100,000 to 1:24,000 (table 1). Raster land-cover source data 

(C-CAP Regional Land Cover and National Land Cover Data 

impervious) have a 30-m resolution. Registration of source 

data after projection to NAD83, UTM Zone 10 N was not 

assured and, thus, introduces a source of uncertainty in the 

assessment. Registration errors would have the greatest effect 

on attributes along the boundaries of floodplains, on river 

banks, and on roads. In some cases (for example, a road that 

occupies a cell with forest land cover), apparent conflicts in 

attributes of the land surface may be due to registration errors 

(road data are not aligned with land cover data), limits of data 

resolution (cell is mostly covered by forest), or physically 

possible configurations (forest canopy covers the road).

Derived Datasets

Geospatial datasets with topographic and hydraulic 

information were derived from the above data sources 

as intermediate steps in the production of assessment 

outputs (table 2). The derivative data include extrapolated 

water‐surface elevations, land forms, and floodplain 

connectivity, which were used throughout the assessment 

and additional characteristics, such as land-surface and 

water‐surface gradients, and land cover in neighboring cells.

Land Forms—Valley Bottoms, High Floodplains, 

Low Floodplains, and River Areas

A modified version of "height above water surface" 

(Jones, 2006) was used to delineate four land forms—valley 

bottoms, high floodplains, low floodplains, and river areas. 

Elevations from the NED for cells crossed by major rivers 

were extrapolated across each river basin. Areas where the 

land surface was less than 10 m above the extrapolated water 

surface were designated as valley bottoms. The other three 

land forms (river areas, low floodplains, and high floodplains) 

were delineated respectively as areas lower than a base 

water surface elevation plus the estimated changes in stage 

from median daily stage for a high flow, small flood, and 

large flood. 

The base water surface elevations (riv_z_ned) used to 

delineate high floodplains were extracted from the elevation 

values in NED for cells crossed by the NHD flowlines for 

the major rivers and extrapolated across the valley bottoms 

using inverse distance weighting. Because of the resolution of 

NED, cells used to extract river elevations may include river 

banks and hillslopes. In these cases, river elevations and the 

extrapolated water surface elevations will have an upward 

bias. The base water surface elevations used to delineate low 

floodplain and rivers were extracted from lidar, because of 

its higher vertical and horizontal resolution, and extrapolated 

across the valley bottoms. Because lidar data were not 

available for all river segments, low floodplain and river areas 

were not delineated comprehensively.
Overview of Assessment 5

Table 1. Source datasets used in the geospatial assessment of ecological functions and flood-related risks on floodplains along 

major rivers, Puget Sound Basin, Washington.

[m, meters; km2, square kilometers]

Dataset description File name Source

Land surface elevation from National Elevation Dataset, in meters; 

10-m raster

land_z_ned U.S. Geological Survey (2012)

Land surface elevation from lidar, in meters; 2-m raster land_z_lidar Puget Sound LiDAR Consortium (2011)

Primary, secondary, local, and private roads; vectors converted to 

10-m raster

vb_roads U.S. Census Bureau (2010)

Major levees and dikes in the Puget Sound basin; polyline vector 

converted to 10-m raster

vb_levees U.S. Army Corps of Engineers (2013); Washington State 

Department of Ecology (2010); and Roger Fuller 

(Western Washington University, written commun., 

February 25, 2014)

Railroads; 1:24,000 vector converted to raster vb_rail Washington Department of Transportation (2013)

Forested areas indicated by codes 9–11 and 13 in the CoastalChange Analysis Program (C-CAP) 2011 Regional Land Cover 

Data Set; 10-m raster

vb_forest National Oceanic and Atmospheric Administration 

(2014)

Wetlands indicated by codes 13–18 in the C-CAP 2011 Regional 

Land Cover Data Set; 10-m raster

vb_wetland National Oceanic and Atmospheric Administration 

(2014)

Undeveloped area indicated by codes 9 through 25 in the C-CAP 

2011 Regional Land Cover Data Set; excludes low to high 

development, cultivated land, pasture/hay, grassland; 10-m 

raster

vb_undev National Oceanic and Atmospheric Administration 

(2014)

Areas with medium or high development (>60 percent impervious 

surface) indicated by codes 2 and 3 in the C-CAP 2011 Regional 

Land Cover Data Set; 10-m raster

vb_mhdev National Oceanic and Atmospheric Administration 

(2014)

Presence of impervious surface in the 2011 National Land Cover 

Dataset, 10-m raster

vb_imperv U.S. Geological Survey (2014)

Land use as designated by tax parcel vb_landuses Washington State Department of Ecology (2010)

Erodible surficial geology including unconsolidated sediments, 

primary Quaternary alluvium, and glacial/lacustrine deposits; 

1:100,000 scale polygons converted to 10-m raster

vb_geoerode Washington Department of Natural Resources (2010)

Hydric rating of SSURGO soil map units; Soil Survey Geographic 

Database polygons (variable scale depending on survey) 

converted to 10-m raster

vb_hydric National Resource Conservation Service (2013)

Volumetric mean fraction silt and clay of SSURGO soil map units; 

Soil Survey Geographic Database polygons (variable scale 

depending on survey) converted to 10-m raster.

vb_siltclay National Resource Conservation Service (2013)

Volumetric mean fraction organic matter of SSURGO soil map 

units; Soil Survey Geographic Database polygons (variable scale 

depending on survey) converted to 10-m raster

vb_om National Resource Conservation Service (2013)

Special Flood Hazard Areas with 1 percent annual probability of 

flooding; 12:000 scale polygons converted to 10-m raster

sfha Federal Emergency Management Agency (2013)

Major rivers and their tributaries with greater than 50 km2 drainage 

area from the high resolution National Hydrography Dataset 

(file: flowline.shp); 1:24,000 polyline vector

PugetRivers.shp U.S. Geological Survey (2013)

Distributions of Chinook, steelhead, and bull trout from the Salmon 

and Steelhead Stock Inventory Assessment Program; 1:24,000 

polyline vector

ESAfishdist.shp Washington Department of Fish and Wildlife (2013)

River segments listed as impaired for temperature, dissolved 

oxygen, or pH (Clean Water Act Section 303d list); 1:24,000 

polyline vector

PugetRiversWQ.shp Washington State Department of Ecology (2013a)

Class A and B water systems, wastewater treatment facilities, 

large septic systems, fire stations, hospitals, emergency medical 

services; points

vb_facilities.shp Washington Department of Health (2013); Washington 

State Department of Ecology (2013b)
6 Geospatial Assessment of Ecological Functions and Flood-Related Risks, Puget Sound Basin, Washington

Table 2. Derived datasets used in the geospatial assessment of ecological functions and flood-related risks on floodplains along 

major rivers, Puget Sound Basin, Washington.

[km2, square kilometers; m, meters; GIS, geographic information system; NED, National Elevation Dataset]

Dataset description File GIS process

Water surface elevation (m) of river from NED 

extrapolated across valley bottom

riv_z_idw_ned Convert major rivers vector (PugetRivers.shp) to raster; create raster of 

NED elevation of river cells; extrapolate water surface elevation of 

rivers across valleys using inverse-distance weighting (IDW).

Water surface elevation (m) of river from lidar 

extrapolated across valley bottom

riv_z_idw_lid Create grid of river cells from lidar data; create grid of lidar elevation 

of river cells; extrapolate water surface elevation of rivers across 

valleys using inverse-distance weighting (IDW).

Valley bottom area where the land surface is less 

than 10 m above water surface elevation of a 

river based on NED

vb Calculate the difference between land surface and extrapolated water 

surface elevations and identify areas less than 10 m above water 

surface using vb = land_z_ned –riv_z_ned < 10 m.

Floodplain units representing floodplain areas 

associated with river valley segments

fpu.shp Merge Puget Watershed Characterization analysis units (Washington 

State Department of Ecology, 2014) to form polygons that spanned 

laterally across floodplains.

Drainage area (km2) of the river in the floodplain 

unit (not the drainage area of a given cell) at 

the downstream end of the floodplain unit

fpu_dr_area Calculate area of floodplain unit polygons, summed downstream, and 

added as an attribute of floodplain units; create a raster where the 

value is the drainage area of floodplain unit.

Nominal stage (m) of a high flow above the 

water surface of the river

dstage_riv Calculate stage increase for a high flow, Dstage_riv = 0.5 × log10 

(vb_dr_area/10), to delineate river areas.

Nominal low flood stage (m) above the water 

surface of the river

dstage_low Calculate stage increase for a small flood, Dstage_low = 1.3 × log10 

(vb_dr_area/10), to delineate low floodplain.

Nominal high flood stage (m) above the water 

surface of the river

dstage_high Calculate stage increase for a large flood, Dstage_high = 2.0 × log10 

(vb_dr_area/10), to delineate high floodplain.

River areas that are likely to include channels 

and off-channel areas

riv_lid Identify river areas based on the relation, Land_z_lid – riv_z_lid < 

dstage_riv.

River areas delineated from lidar or from NHD 

flow lines

rivmerg Merge river areas delineated from lidar cells crossed by high resolution 

NHD flow lines, to represent rivers outside of lidar coverage.

Areas adjacent to but outside of connected river 

areas representing river banks;

rivmerg_bank Select cells outside of river area where at least one neighboring cell is 

in rivmerg.

River areas at the edge of the river rivmerg_edge Select cells in rivermerg where at least one neighboring cell is outside 

of rivmerg.

Distance (m) to nearest river cell rivmerg_dist Calculate minimum distance to rivmerg.

Low floodplain delineated from lidar fp_low_lid Identify low floodplain delineated from lidar based on the relation, 

land_z_lid – riv_z_lid < dstage_low.

High floodplain delineated from NED fp_high_ned Identify high floodplain delineated from NED based on the relation, 

land_z_ned – riv_z < dstage_high.

Roads, levees, and railroads frag_lines Combine roads, levees, and railroads into a single grid, all features 

have an arbitrary width of one cell (10 m).

Low floodplain fragments fp_low_frag.shp Divide low floodplain into polygons using roads, levees, or railroads, 

add attribute “Connected” and assign a value of 1 if the polygon is 

crossed by a river segment or 0 if there is no river segment crossing 

the polygon.

High floodplain fragments fp_high_frag.shp Divide high floodplain into polygons using roads, levees, or railroads, 

add attribute “Connected” and assign a value of 1 if the polygon is 

crossed by a river segment or 0 if there is no river segment crossing 

the polygon.

Floodplain areas categorized by connectivity and 

land form

fp Combine rivmerg, fp_low_frag, fp_high_frag; assign values 3 = river 

areas (rivmerg = 1), 2 = connected low floodplain (fp_low_frag = 

1), 1 = connected high floodplain (fp_high_frag = 1), 0 = valley 

bottom outside of floodplain or road/levee/railroad, -1 = reduced 

connectivity floodplain.
Overview of Assessment 7

Table 2. Derived datasets used in the geospatial assessment of ecological functions and flood-related risks on floodplains along 

major rivers, Puget Sound Basin, Washington.—Continued

[km2, square kilometers; m, meters; GIS, geographic information system; NED, National Elevation Dataset]

Dataset description File GIS process

Maximum gradient (slope) of interpolated river 

surface 

riv_grad_ned Calculate the maximum gradient (slope) of extrapolated water surface 

elevation, riv_z_idw_ned, calculated over a 250-m length scale.

Undeveloped area in neighborhood undev_fsum Count the number of undeveloped cells in the 3 × 3 (30 m × 30 m) 

neighborhood.

Lowest land form in neighborhood fmax Identify the lowest elevation landform (river, low floodplain, or high 

floodplain) in the 3 × 3 (30 m × 30 m) neighborhood.

Percent of drainage area that is not regulated by 

major reservoir;

reg_frac Calculate the percentage of drainage area that is not regulated by major 

reservoir.

Areas likely to have shallow groundwater gw_shallow Identify areas less than 1 m above the extrapolated water surface 

elevation of the river (riv_z_idw_ned).

Slope of land surface in percent (1 percent = 1). ps_slope Calculate the slope of land surface, in percent (1 percent = 1).

0

1

2

3

4

5

6

10 100 1,000 10,000

Change in stage, dz, in meters

Drainage area, in square kilometers

Increase in annual maximum stage above median daily stage

High floodplain threshold: dz=2.0*log(drainage area/10)

Low floodplain threshold: dz=1.3*log(drainage area/10)

River Area threshold: dz=0.5*log(drainage area/10)

EXPLANATION

90th percentile of annual maximum

50th percentile of annual maximum

Figure 2. Change in stage from median daily stage for the 50th and 

90th percentiles of annual maximum stage as a function of drainage 

area at 40 U.S. Geological Survey streamgaging stations on unregulated 

rivers without levees with at least 10 years of daily stage records, Puget 

Sound Basin, Washington. Lines represent the change in stage used for 

delineating river areas, low floodplains, and high floodplains.

Relations for the increase in stage for 

a high flow, small flood, and large flood as 

a function of drainage area were derived 

from daily stage records from 40 U.S. 

Geological Survey streamgaging stations 

on the unregulated river in sections without 

levees in the Puget Sound Basin with at least 

10 years of record. The increase from median 

daily stage to median annual maximum 

stage ranged from 0.34 to 3.4 m depending 

on the drainage area of the site (fig. 2). 

The increase from median stage to the 90th 

percentile of annual maximum stage ranged 

from 0.5 to 4.2 m. Both of these statistics 

generally increased with drainage area, so 

simple relations were derived to estimate the 

increase in stage (dstage_riv, dstage_low, 

and dstage_high) for a high flow event (lower 

envelope of median annual maximum stage 

increase), a small flood (upper envelope of 

median annual maximum stage increase), 

and a large flood (upper envelope of the 90th 

percentile of annual maximum stage increase) 

as a function of the base-10 logarithm (log10) 

of drainage area:

• dstage_riv = 0.5 log10 (drainage 

area/10);

• dstage_low = 1.3 log10 (drainage 

area/10); and

• dstage_high = 2.0 log10 (drainage 

area/10).
8 Geospatial Assessment of Ecological Functions and Flood-Related Risks, Puget Sound Basin, Washington

The increase in stage for high flows, small floods, and 

large floods were calculated for each cell in the valley bottom 

raster based on the drainage area of closest river. Areas where 

the difference between the land-surface elevation (land_z_lid

or land_z_ned depending on the source of elevation data) and 

interpolated river-surface elevation (riv_z_lid and riv_z_ned) 

are less than the respective increase in stage for each land 

form are assigned to that land form:

• river area, land_z_lid – riv_z_lid < dstage_riv. 

• low floodplain, land_z_lid - riv_z_lid < dstage_low; 

and

• high floodplain, land_z_ned – riv_z_ned < dstage_

high.

Delineation of floodplains using the elevation of the 

land surface relative to an estimated water surface elevation 

based on drainage area was determined to be the best feasible 

method that could be applied consistently across the Puget 

Sound Basin. Because the estimated water surface elevations 

are only approximate, the floodplains delineated from this 

method have significant caveats. The floodplains do not 

have specific probability of inundation. Areas higher than 

high floodplains may be inundated during floods, occupied 

by rivers, or provide ecological functions associated with 

floodplains. The floodplains delineated for this assessment 

should not be used in any application as an alternative to 

Special Flood Hazard Areas (Federal Emergency Management 

Agency, 2013).

Connectivity

The movement of water, biota, sediment, wood, 

and other materials between rivers and floodplains is a 

fundamental requirement for many ecological functions of 

floodplains (Ward and Stanford, 1995; Ward and others, 2002). 

Floodplains are ecologically disconnected by human actions 

that limit the frequency, duration, or types of connection 

between the floodplain and a river even if that floodplain is 

still inundated during some floods. Flood regulation, bank 

revetments, levees, and roads disconnect floodplains from 

rivers by reducing the movement of materials and biota 

or limiting the movement to a portion of the floodplain. 

Floodplain connectivity depends on the elevation of the 

land surface relative to river stage (as discussed for land 

forms) but also on paths between a floodplain and a river for 

water, sediment, organic material, and biota, which include 

small surface distributaries and subsurface zones with 

high permeability. 

Connectivity of floodplains was assessed using 

topography and infrastructure that are likely to impede the 

movement of materials or biota. As the first step, any cells 

crossed by roads, railroads, and levees were removed from 

the floodplain. This step presumes these features have a 

nominal width of 10 m and provide no ecological function. 

The remaining floodplain cells were converted into polygons 

where each polygon had a border defined by a road, railroad, 

levee, or land surface higher than the high floodplain. The 

resulting floodplain polygons are referred to as “fragments.” 

Fragmentation of floodplains directly from the vector 

centerlines of roads, railroads, and levees is an alternative, but 

computationally intensive, approach that nonetheless requires 

a presumption about the width of these features to close gaps 

between mapped centerlines where these features actually abut 

each other.

Any floodplain fragment crossed by a river was 

designated as a “connected” floodplain. Fragments that were 

separated from a river by topography, roads, railroads, or 

levees were designated as a “reduced connectivity” floodplain 

indicating some type of disconnection is probable. Under 

this scheme, a floodplain fragment had to be completely 

surrounded by road, railroad, levee, or high elevation land 

to have reduced connectivity. Fragments separated from a 

river by a low road or railroad trestle, for example, would be 

considered to have reduced connectivity even as some level 

of connectivity may be maintained through these structures. 

Conversely, a connected fragment can include areas behind 

roads or levees that have small gaps even as connectivity 

would likely be impaired by the road or levee. Further 

refinement of connectivity requires site-specific information 

beyond the scope of the assessment. 

Fine-Resolution Assessment

For the fine-resolution assessment, each type of function, 

hazard, or exposure was assessed in terms of attributes of 

10-m cells covering the valley bottoms for all major rivers. 

Each type of function (fn), hazard (haz), or exposure (exp) 

has a single primary output file in the fine-scale assessment 

designated as fn1, fn2, fn3, fn4, fn5, haz1, haz2, exp1, exp2, 

or exp3. A letter following the primary number (for example, 

fn1a) indicates a secondary output providing additional 

information about a specific aspect of the function, hazard, or 

exposure. A summary of the GIS models and output values for 

all functions, hazards, and exposures are provided in table 3. 

The output datasets are available online as part of this report.
Fine-Resolution Assessment 9

Table 3. Summary of models and outputs used for fine-resolution (10-meters) assessment of floodplain function, hazard, and exposure, 

Puget Sound Basin, Washington.

[km2, square kilometers; GIS, geographic information system]

Function, Hazard, or Exposure

GIS Model

Output

Floodplain functions

1. Store and convey floods

fn1 = fp * vb_undev - 2 * (fp > 0) * (1 - vb_mhdev)*(1-vb_undev)

3: connected, undeveloped river area

2: connected, undeveloped low floodplain

1: connected, undeveloped high floodplain

0: roads, levees, or railroads; not floodplain

-1: reduced connectivity, undeveloped floodplain

-2: connected floodplain with low development (less than medium/high)

1a. Store floodwater

fn1a = (dstage_high – dz_ned) * (fp > 0) – (fp == -1)

>0: depth of water during a nominal flood, in meters

0: roads, levees, or railroads; not floodplain

-1: reduced connectivity floodplain

1b. Convey floods

fn1b = (fp > 1) * (vb_undev) * (riv_grad_ned)

>0: hydraulic gradient [m/m] based on National Elevation Dataset (U.S. Geological Survey, 2012) for river areas and low floodplains

0: areas outside of lidar coverage; high or reduced connectivity floodplain; road/levee/railroad; not floodplain

2. Regulate sediment and wood supplies in river networks

fn2 = vb_geoerode * ((vb_imperv == 0)* IsNull(vb_fraglines)* (rivmerg_bank * (vb_forest + vb_undev + 2 - (vb_undev==0) * 3) + 

(rivmerg_bank == 0) * (fp > 0) * (vb_forest + vb_undev - (vb_undev==0) * 2) - (rivmerg_bank == 0) * (fp == -1) * (vb_undev * 3 + 

(vb_undev==0) * 4)) - ((vb_imperv + IsNull(vb_fraglines)==0) > 0) * rivmerg_bank * 5)

All areas with values other than 0 are composed of unconsolidated material.

4: forest areas adjacent to river with no impervious surfaces

3: undeveloped, non-forested areas adjacent to river with no impervious surfaces

2: forested, connected floodplain with no impervious surfaces

1: undeveloped, non-forested connected floodplain with no impervious surfaces

0: floodplain on consolidated material; roads, levees, or railroads not adjacent to river areas; not floodplain

-1: developed areas adjacent to river; no impervious surfaces 

-2: developed, connected floodplain; no impervious surfaces

-3: undeveloped reduced connectivity floodplain, no impervious surfaces

-4: developed reduced connectivity floodplain; no impervious surfaces

-5: areas adjacent to river with impervious surfaces, roads, levees, or railroads

2a. Supply sediment

fn2a = rivmerg_bank * (dz_ned) * (dz_ned>0) * IsNull(vb_frag_lines) * (1-vb_imperv)

>0: Height [m] of areas adjacent to river with no impervious surfaces

0: roads, railroads, or levees; river area; areas not adjacent to a river

2b. Retain sediment and particulate organic material

fn2b = (fp > 0) * riv_grad_ned * vb_undev

>0: hydraulic gradient (m/m) for undeveloped, connected floodplain

0: reduced connectivity or developed floodplain; not floodplain
10 Geospatial Assessment of Ecological Functions and Flood-Related Risks, Puget Sound Basin, Washington

Table 3. Summary of models and outputs used for fine-resolution (10-meters) assessment of floodplain function, hazard, and exposure, 

Puget Sound Basin, Washington.—Continued

[km2, square kilometers; GIS, geographic information system]

Function, Hazard, or Exposure

GIS Model

Output

Floodplain functions—Continued

3. Retain and transform nutrients and contaminants

fn3 = ((fp > 0) * (vb_imperv == 0) – (fp ==-1)) * (ps_slope) /100

>0: land surface gradient [m/m] for connected floodplain with no impervious surface

0: floodplain with impervious surface; road, railroad, or levee; not floodplain

<0: land surface gradient for reduced connectivity floodplain with no impervious surface

3a. De-nitrification

fn3a = ((vb_wetland + gw_shallow + vb_hydric) > 0.5) * (vb_imperv == 0) * (1+(vb_siltclay>0.3)*((vb_siltclay > 0.3) + 

(vb_om > 0.02)))

3: valley bottom with anaerobic indicator (wetland, shallow groundwater, or hydric soils), silt/clay > 0.3, and organic matter >0.02, 

and no impervious surfaces

2: valley bottom with anaerobic indicator; silt/clay > 0.3, organic matter < 0.02, and no impervious surfaces

1: valley bottom with anaerobic indicator; silt/clay < 0.3, organic matter < 0.02, and no impervious

0: no anaerobic indicator; impervious surfaces

3b. Retain and transform contaminants

fn3b = (rivmerg==0) * (vb_siltclay > 0.3) * (vb_undev) * (9 - vb_undev_fsum) - vb_mhdev

1–8: number of neighboring cells with development (urban, residential, agricultural, or other commercial land use) adjacent to an 

undeveloped cell on the valley bottom with silt/clay soils; 1 indicates that one of the 8 neighboring cells are developed; 

8 indicates that all neighboring cells are developed

0: none of the neighboring cells are developed; river area

-1: developed areas of the valley bottom

4. Support forest ecosystems

fn4 = (vb_imperv == 0) * (vb_undev) * ((fp > 0) - 2 * (fp < 0))+ ((vb_forest + vb_wetland)>0)

2: connected floodplain with forest/wetland cover; no impervious surfaces

1: connected floodplain, undeveloped; no impervious surfaces

0: developed floodplain; impervious surfaces; not floodplain; 

-1: reduced connectivity floodplain with forest/weltand; no impervious surfaces

-2: reduced connectivity floodplain, undeveloped; no impervious surfaces

5. Provide aquatic habitats

fn5 = (riv10m + riv10m_edge) * vb_undev * IsNull(vb_frag_lines) * (vb_imperv == 0)

fn5_merg = (rivmerg + rivmerg_edge) * vb_undev * IsNull(vb_frag_lines) * (vb_imperv == 0)

2: undeveloped river edge areas with no roads, railroads, levees, or impervious surfaces

1: undeveloped river areas with no roads, railroads, levees, or no impervious surfaces

0: areas above river stage; developed areas; areas with road, railroads, or levees, impervious surfaces

Flood-related hazards

1. Inundation

haz1 = sfha * fp – 2*(sfha == 0) * (fp ≠ 0)

3: connected river areas in special flood hazard area (SFHA)

2: connected low floodplain in SFHA (areas likely to be inundated during floods)

1: connected high floodplain in SFHA (areas that may be inundated during major floods)

0: neither SFHA nor floodplain

-1: reduced connectivity floodplain in SFHA (areas dependent on infrastructure to reduce flooding)

-2: floodplain out of SFHA (areas that may depend on infrastructure to prevent floods)
Fine-Resolution Assessment 11

Table 3. Summary of models and outputs used for fine-resolution (10-meters) assessment of floodplain function, hazard, and exposure, 

Puget Sound Basin, Washington.—Continued

[km2, square kilometers; GIS, geographic information system]

Function, Hazard, or Exposure

GIS Model

Output

Flood-related hazards—Continued

2. Channel occupation

haz2 = vb_geoerode * (fp ≠ 0) * (1 + (rivmerg_dist < (10*SquareRoot(fpu_dr_area))) * (1 + (rivmerg_dist < SquareRoot(fpu_dr_area)) 

+ 3 * (fpu_grad > 0.002) + 3 * (fpu_grad > 0.005) + ((slp30dSQKM/fpu_dr_area) > 0.2) * (fpu_grad > 0.002) * (rivmerg_dist < 

SquareRoot(fpu_dr_area))))

10: erodible areas close to high gradient rivers with high sediment loads

9: erodible areas close to high gradient rivers with moderate sediment loads

8: erodible areas near to high gradient rivers

7: erodible areas close to intermediate gradient rivers with high sediment loads

6: erodible areas close to intermediate gradient rivers with moderate sediment loads

5: erodible areas near to intermediate gradient rivers

3: erodible areas close to low gradient rivers

2: erodible areas near to low gradient rivers 

1: erodible areas on floodplain faraway from rivers

0: floodplains on consolidated geology; roads, railroads, or levees

Proximity scales with drainage area. The distance from a “close” areas to a river is less than square root of drainage area of the associated 

river (within 10 m of a river with a 100 km2 drainage area, 100 m for a river with 10,000 km2 drainage area). The distance from a 

“near” areas to a river is greater than the square root of drainage area of the associated river but less than 10 times the square root of 

drainage area of the associated river (between 10 and 100 m for a river with a 100 km2 drainage area and between 100 and 1,000 m for 

a river with a 10,000 km2 drainage area).

Sediment loads are considered high if more than 20 percent of a river’s drainage area is unregulated with slopes greater than 30 degrees.

River gradients greater than 0.005 are steep, 0.002 to 0.005 are intermediate, and less than 0.002 are low.

2a. Bank erosion

haz2a = vb_geoerode * rivmerg_bank * (vb_siltclay < 0.3) * (fpu_sinuous > 11) * (1 + (fpu_sinuous > 14) + 2 * (fpu_grad>0.002) + 2 * 

(fpu_grad>0.005))

6: erodible banks along a steep, meandering river

5: erodible banks along a steep, sinuous river

4: erodible banks along an intermediate gradient, meandering river

3: erodible banks along an intermediate gradient, sinuous river

2: erodible banks along a low gradient, meandering river

1: erodible banks along a low gradient, sinuous river

0: not river bank or river bank formed from consolidated geologic material

Meandering rivers have a sinuosity > 1.5, sinuous rivers have a sinuosity from 1.2 to 1.5.

2b. Avulsion

haz2b =(fp> 1) * (1 + (riv_grad_ned > 0.002) +(riv_grad_ned > 0.005)) * (1 + 2 * ((slp30dSQKM/fpu_dr_area) > 0.2))

5: river and low floodplain areas along high gradient rivers with high sediment loads

4: river and low floodplain areas along high gradient rivers with moderate sediment loads

3: river and low floodplain areas along intermediate gradient rivers with high sediment loads

2: river and low floodplain areas along intermediate gradient rivers with moderate sediment loads

1: river and low floodplain areas along low gradient rivers

0: high floodplain; road, railroad, or levee; or not floodplain
12 Geospatial Assessment of Ecological Functions and Flood-Related Risks, Puget Sound Basin, Washington

Function, Hazard, or Exposure

GIS Model

Output

Flood-related exposure

1. Key facilities

exp1 = vb_facilities * (10* (fp >10) + (fp == 1) – (fp == -1))

0: no key facility

10, 1, -1: water system on low floodplain, high floodplain, and reduced connectivity floodplain, respectively

20, 2, -2: wastewater discharge on low floodplain, high floodplain, and reduced connectivity floodplain, respectively

30, 3, -3: large septic system on low floodplain, high floodplain, and reduced connectivity floodplain, respectively

40, 4, -4: fire station on low floodplain, high floodplain, and reduced connectivity floodplain, respectively

50, 5, -5: hospital on low floodplain, high floodplain, and reduced connectivity floodplain, respectively

60, 6, -6: emergency medical service facility on low floodplain, high floodplain, and reduced connectivity floodplain, respectively

2. Land uses

exp2 =((fp > 0) - (fp == -1)) * ((vb_landuse == 7) +(vb_landuse == 76) + (vb_landuse ==85) + (vb_landuse == 87) + (vb_landuse == 1) * 

2 + (vb_landuse == 2) * 3 + (vb_landuse == 5) * 3 + ((vb_landuse == 6) * 3 + (vb_landuse == 4) * 4 + (vb_landuse == 81) * 5)

5: agriculture

4: transportation/communication/utility

3: manufacturing, trade, services

2: residential

1: other (cultural/entertainment/recreation; parks, mining, timber)

0: undeveloped, reduced connectivity floodplain, or above floodplain

3. Roads

exp3 = fp_focmax, vb_roads as mask

3: road that border river area

2: road that border connected low floodplain

1: road that border connected high floodplain

0: not road; road not adjacent to river or floodplain

Table 3. Summary of models and outputs used for fine-resolution (10-meters) assessment of floodplain function, hazard, and exposure, 

Puget Sound Basin, Washington.—Continued

[km2, square kilometers; GIS, geographic information system]

The outputs from the fine-resolution assessment generally 

are categories of function or risk where each category 

represents a unique combination of attributes (for example, 

connected floodplain with impervious surface). Each output 

category is intended to represent functionally distinct types 

of floodplains, but the value assigned to the category does 

not represent the degree of function. In some cases, areas 

with the highest value may be expected to provide the most 

function and areas with the lowest value may be expected to 

provide the least function. This convention does not hold in 

many cases, so the degree of function should not be presumed 

from the order of output values. For example, the output 

values for a disconnected floodplain with forest cover and a 

connected floodplain with development are not intended to 

indicate whether disconnection or land cover conversion has a 

greater influence on a function. The values are only intended 

to indicate that these floodplains likely have functional 

differences. Zero values are reserved for areas where the 

function, hazard, or exposure is unlikely because of either 

biophysical or social factors. Negative values are reserved 

for areas that may not provide a function but may have 

potential for improvement (for example, reduced connectivity 

floodplain with forest cover) or that represent a distinct class 

(for example, reduced connectivity floodplain) where the 

output is a continuous variable.

Floodplain Functions

Function 1. Store and Convey Floods

The defining characteristic of floodplains is their capacity 

to store and convey water during floods. This function depends 

on areas that have a lower elevation than flood stage and 

are connected to a river. In addition, floodplains that convey 

floodwater must be relatively free of obstructions that impede 
Fine-Resolution Assessment 13

flowing water. The six output categories for this function 

represent combinations of the relative elevation of floodplain 

areas (low or high floodplain), connectivity (connected and 

reduced connectivity), and level of development, which may 

indicate the presence of structures that could interfere with this 

function. Floodplains with roads, levees, railroads, or medium/

high levels of development are assigned an output value of 0 

because they are not anticipated to store or convey significant 

volumes of water during floods and generally are not managed 

to provide this function during floods.

Key assumptions/uncertainties.—The delineation of 

floodplains in this assessment produces coarse boundaries 

and resulting floodplains have neither a specific nor uniform 

probability of inundation. The floodplains are likely to 

include most areas inundated during frequent floods and 

some areas that are inundated infrequently. Roads, railroads, 

and levees are assumed to disconnect floodplain areas from 

rivers only when they form a continuous boundary around 

floodplain areas. In some cases, floodplains may have reduced 

connectivity even if roads, levees, and railroads do not form a 

continuous boundary around them. Conversely, road, railroads, 

and levees may not be effective barriers to inundation during 

some floods.

Function 1a. Store Floodwater

Floodplains store water during floods, which maintain 

soil moisture, groundwater recharge, and temporary aquatic 

habitats (Ward and others, 2002). The storage volume 

provided by a floodplain area depends on the elevation of the 

land surface relative to flood stage. The difference between 

the land-surface elevation and flood stage can be used as an 

approximate index of the storage capacity of a floodplain. 

The output for this function is the depth of water (continuous 

values with units of meters) at a given location for a nominally 

high flood stage. Reduced connectivity floodplains are 

assigned a value of -1. Floodplains with roads, levees, or 

railroads are assigned an output value of 0 for this function.

Key assumptions/Uncertainties.—The change in stage 

expected for a high flood is based on the relation between the 

changes in stage for the 90th percentile flood at streamgaging 

stations in reaches without levees. Streamgaging stations 

typically are located in more confined reaches, so the change 

in stage at these locations would have an upward bias 

compared to reaches in unconfined floodplains. All roads, 

levees, and railroads are treated as barriers to flooding without 

discriminating at what river stage they actually provide a 

barrier to flooding. This methodology does not represent 

local variation in flood discharge and stage and should not be 

considered an estimate of flood depth at any location.

Function 1b. Convey Floods

Floodplains convey water during floods, which reduces 

the spatial extent and depth of flood inundation in surrounding 

areas (Konrad and others, 2008). The capacity of floodplains 

to convey floods depends primarily on their width, hydraulic 

gradient, which can be approximated by the water-surface 

gradient and roughness (Chow, 1959). In most floods, only 

the river channel and low adjacent floodplains convey water, 

so the assessment of this function is limited to connected 

low floodplain and river areas. Roughness is a measure of 

flow resistance and comes from various sources including 

meander bends, gravel bars, vegetation, debris, and the surface 

material of a river channel or floodplain (Knighton, 1998). 

Most of the major rivers in the Puget Sound Basin are wider 

than 10 m, so width is addressed in the coarse-resolution 

assessment. The output values for this function are hydraulic 

gradient (continuous values with units of meters per meter) 

for connected river and low floodplain that are likely to 

have low flow resistance. Areas outside of lidar coverage, 

reduced connectivity floodplains, floodplains with low to high 

development, forests, roads, levees, or railroads are assigned 

an output value of 0 for this function.

Key assumptions/Uncertainties.—River areas and 

low floodplains are more important than high floodplains for 

conveying floods. River areas were delineated only where 

lidar data were available. River gradients calculated from the 

interpolated river surfaces over a length of 250 m may not 

represent hydraulic gradients of rivers at flood stage, such as 

where local channel morphology or debris forms constrictions. 

In reaches with uniform widths and flow resistance, however, 

hydraulic gradients can be expected to become more uniform 

at increasing stage. Forests and developed areas may convey 

floodwaters, but are assumed to contribute substantially less 

conveyance capacity than other areas. Areas likely to have low 

flow resistance include open space, cultivated land, pasture, 

grassland, scrub/shrub including wetland, bare land, and water 

(C-CAP codes 5, 6, 7, 8, 12, 14, 15, 17, 18, 19, 20, and 21). 

Areas with low to high development or forest of any kind 

(codes 2, 3, 4, 9, 10, 11, 12, 13, and 16) are likely to have 

higher flow resistance.

Function 2. Regulate Sediment and Wood 

Supplies in River Networks 

Floodplains regulate fluvial transport of sediment and 

large woody debris delivered from headwater streams and 

hillslopes through river networks (Fetherston and others, 1995; 

Knighton, 1998). These materials are deposited on floodplains, 

contributing to the formation of aquatic and terrestrial habitats, 

and may be eroded later, serving as a proximate supply of 

these materials to downstream reaches. The capability of 

areas to store and supply sediment to river networks during 

floods depends on their accessibility by a river, land cover, and 

surficial geology. The 10 output categories for this function 

incorporate forest cover, erodible materials, proximity to 

rivers, and connectivity. Areas that are on consolidated 

materials (bedrock) and roads, railroads, or levees are assigned 

an output value of 0 for this function.
14 Geospatial Assessment of Ecological Functions and Flood-Related Risks, Puget Sound Basin, Washington

Key assumptions/Uncertainties.—River banks and 

floodplains are proximate source of sediments for fluvial 

transport. Hillslopes above floodplains are a source of 

sediment addressed under Function 2a. Unconsolidated 

materials (for example, sand, gravel) are much more erodible 

than bedrock, other consolidated material, and clay. Areas 

with impervious surfaces, road, levees, and railroads are likely 

to be protected by revetments to protect the infrastructure 

and generally do not supply sediment or wood to channels. 

Developed areas can include some trees, but the supply of 

large woody debris would be limited from these areas. River 

areas were delineated only where lidar data were available. 

Forest cover classes do not distinguish age, size, or density of 

trees, which influence the value of the forest for contributing 

large woody debris to a river network. Conifer, deciduous, 

and mixed forest types were considered equivalent for 

supplying wood.

Function 2a. Supply Sediment

Floodplains and hillslopes along river channels are the 

local supply of sediment to rivers (Knighton, 1998). Although 

river banks supply sediment to rivers when they are eroded 

during high flows, steep, high hillslopes deliver sediment to 

rivers and floodplains through landslides and other forms of 

mass wasting. Revetments often are used to protect roads, 

railroads, levees, and other infrastructure in close proximity 

to rivers by preventing erosion. The output values for this 

function are the height of areas adjacent to rivers (continuous 

value with units of meters) without roads, levees, railroads, 

or other impervious surface. Floodplain areas that are not 

adjacent (neighboring cell) to rivers are assigned an output 

value of 0 for this function.

Key assumptions/Uncertainties.—Margin height 

indicates the potential volume of sediment delivered to 

a channel through bank erosion. Consolidated materials 

including bedrock can be an important source of sediment 

where those materials form high bluffs adjacent to rivers. The 

erodibility of material is not considered in this assessment 

but does influence the sediment supply rate to rivers. River 

margins were delineated only where lidar data where 

available. River margins areas with infrastructure likely do not 

supply sediment because of revetments.

Function 2b. Retain Sediment and Particulate 

Organic Material

Floods can deposit sediment, wood, and other particulates 

on floodplains in areas where flow velocities are low enough 

to allow deposition. The hydraulic gradient of the river 

indicates the velocity of water and its competence to transport 

sediment and other particulates. Floodplains with reduced 

connectivity may not be inundated as frequently and, thus, will 

not receive sediment and organic material inputs. The output 

values are the hydraulic gradient of the river in undeveloped 

areas of connected floodplains. Reduced connectivity or 

developed floodplains are assigned an output value of 0 for 

this function.

Key assumptions/Uncertainties.—Floodplains along 

low gradient rivers are more likely to retain particulate 

materials than those along high gradient rivers. Retention 

time is inversely related to hydraulic gradient. The retention 

of sediment is only assessed for undeveloped floodplains, 

although agricultural lands or developed open space also may 

retain sediment during floods.

Function 3. Retain and Transform Nutrients and 

Contaminants 

Floodplains retain dissolved materials, including 

nutrients and contaminants that constitute water pollutants, 

where water is stored on floodplains and sorbed to soil 

particles (Marcus, 1989; Baldwin and Mitchell, 2000; Pinay 

and others, 2002). Transformations depend on biophysical 

conditions (light, reduction-oxidation potential, availability of 

co-reactants/microbes, pH, etc.) specific to each constituent, 

but generally lower gradient floodplain areas will have longer 

retention times and fine-grained soils that promote retention 

and biogeochemical transformations (Smith and others, 1988; 

Pinay and others, 2000). Areas with impervious surfaces are 

unlikely to retain water pollutants because of limited contact 

between floodwaters and soil. The output values for this 

function are land surface gradient for connected floodplain 

with no roads, levees, railroads, or other impervious surfaces, 

and land surface gradient for reduced connectivity floodplain 

with no roads, levees, railroads, or other impervious surfaces.

Key assumptions/Uncertainties.—Low gradient 

floodplains that are inundated frequently are most effective at 

retaining pollutants. Soil data generalize river areas as coarsegrained alluvium without distinguishing sand and finer grained 

facies along rivers, so land surface gradient may be a more 

reliable indicator of fine-grained material in river areas where 

soils are mapped as alluvium. Although floodplains retain 

nutrients and contaminants sorbed to fine sediments, these 

pollutants can be resuspended during subsequent flooding.

Function 3a. Denitrification

Floodplains can act as zones of denitrification where 

soil microbes transform nitrate into molecular nitrogen under 

anaerobic conditions (Pucket and others, 2008; Tesoriero and 

Puckett, 2011). Organic carbon serves as the electron donor 

and nitrate as the electron acceptor in denitrification. Nitrate 

is only formed under aerobic condition, so denitrification can 

only occur where there is spatial or temporal transition from 

aerobic to anaerobic conditions in the soil column (Baldwin 

and Mitchell, 2000). Denitrification is associated with high 

silt and clay content, which was greater than 65 percent for 

areas of denitrification on one floodplain (Pinay and others, 

2000). This function is assessed in terms of the primary three 
Fine-Resolution Assessment 15

denitrifying conditions (anaerobic soils, fine-grained sediment, 

and organic carbon) where the likelihood of denitrification is 

highest in the presence of all three conditions. 

Key assumptions/Uncertainties.—Nutrient sources 

are not assessed, but are assumed to be uniformly available 

across the valley bottom. Retention time, contact with soil, 

and the juxtaposition/ transition from anaerobic and aerobic 

environments are the primary factors limiting denitrification 

in river floodplains. Wetlands, shallow groundwater, or hydric 

soils indicate the likelihood for anaerobic conditions, but not 

the transitions between aerobic and anaerobic conditions. 

Fine-grained substrate provides the necessary surface area 

and retention time for denitrification. Despite a mechanistic 

understanding of denitrification, the key factors may not 

account for field measurements of floodplain denitrification 

(Orr and others, 2007). As a result, denitrification is highly 

uncertain even in areas classified as most likely to support 

this function.

Function 3b. Retain and Transform Contaminants

Floodplains may be able to retain and transform some 

contaminants (Marcus, 1989). This capacity depends on 

sources of contaminants in proximity to floodplains areas 

with long retention times (weeks) and soils that will sorb the 

contaminants (Smith and others, 1988), which will promote 

retention or transformation, or both. Potential sources of 

contaminants in this assessment include urban, agricultural, 

and residential land uses. Contaminants are transformed 

through aerobic and anaerobic processes, so no distinction 

is made for this function based on floodplain elevation. The 

output values for this function are the number of neighboring 

cells (0–8) with development (sources of contaminants) for 

each undeveloped cell with high silt/clay. Floodplain areas 

with development are assigned an output value of -1 to 

represent potential sources of contaminants.

Key assumptions/Uncertainties.—Fine-grained 

soil provides sufficient surface area and contact time for 

transformations, whereas coarse-grained soils do not. River 

areas are likely to transport contaminants rather than retain and 

transform them, although some contaminants may deposit with 

fine-grained sediments in low velocity river areas. Developed 

areas are unlikely to retain or transform contaminants except 

where stormwater controls are in place. In these cases, 

however, retention is not solely a function of floodplains. 

Transformation of contaminants is highly uncertain even in 

areas most likely to support this function.

Function 4. Support Forest Ecosystems

Floodplains support productive and diverse forest 

ecosystems in areas where trees can grow to maturity, 

rivers deposit sediment that can be colonized by early seral 

stage plants, and where they are not fragmented by human 

development (Naiman and others, 2010). The five output 

categories for this function represent combinations of 

connectivity and forest/wetland cover. Areas with impervious 

surfaces or other development including agriculture are 

assigned an output value of 0 for this function.

Key assumptions/Uncertainties.—Dominant vegetation 

type (for example, trees, shrubs, grass) is the primary factor 

in determining whether floodplains support forest ecosystems. 

Undeveloped areas without forest or wetland cover have 

the potential to support forests. Other factors, such as soils, 

trees species and age structure, canopy heights and openings, 

and patch size, are not considered. Floodplain forests need 

to be connected to rivers to be fully functional, but forests 

on floodplains with reduced connectivity can support 

some functions.

Function 5. Provide Aquatic Habitats

Floodplains provide a variety of aquatic habitats with 

distinct hydroperiods, depths, velocities, and substrates (Ward 

and others, 2002). Among these, shallow, low velocity water 

along the edges of river-channel and off-channel areas is a 

preferred habitat for rearing of juvenile salmonids (Beechie 

and others, 2005). The two output categories indicating this 

function are undeveloped river edges and undeveloped river 

areas. River areas with roads, levees, railroads, or other 

development and floodplain areas above rivers are assigned an 

output value of 0 for this function.

Key assumptions/Uncertainties.—In areas without lidar 

coverage, river areas were defined as cells crossed by river 

segments from the National Hydrography Dataset (NHD) high 

resolution hydrography. In these areas, river areas are only 

one cell wide and, as each river cell as non-river neighbors, 

all river cells are designated as edge. Land-cover data have a 

30-m resolution and, thus, should only be considered a coarse 

indicator of land cover in the vicinity of river edges.

Flood-Related Hazards

Hazard 1. Inundation

Inundation of land area during floods represents a hazard 

to people using floodplains. Areas with an annual probability 

of inundation greater than 0.01 are designated as Special 

Flood Hazard Areas (Federal Emergency Management 

Agency, 2013). Within SFHA, the probability of flooding 

is likely to depend on the elevation of the land surface and 

connectivity with rivers, so the output for this hazard defines 

four categories of SFHA corresponding to river areas, 

low floodplain, high floodplain, and reduced connectivity 

floodplain. Inundation hazard with an annual probability less 

than 0.01 may be present outside of SFHA in higher elevation 

areas of the floodplain or in areas protected by infrastructure. 

Such areas are a fifth output category. Areas outside of the 

SFHA and floodplain are assigned an output value of 0 for 

this hazard.
16 Geospatial Assessment of Ecological Functions and Flood-Related Risks, Puget Sound Basin, Washington

Key assumptions/Uncertainties.—The distinction 

of inundation hazard within SFHA is only approximate. 

Hydraulic modeling incorporating site-specific information 

would be needed to estimate the probability of inundation 

of floodplain areas. River and low floodplain areas have not 

been delineated in areas without lidar coverage. Because of 

the methods used for delineating floodplain, floodplain areas 

outside of SFHA may have a low inundation hazard.

Hazard 2. Channel Occupation

Rivers change their position on floodplains as a result 

of progressive bank erosion and deposition or a rapid shift of 

flow (or avulsion) to a new location. The shifting occupation 

of floodplains by rivers represents a hazard to many uses of 

floodplains and, as a result, this hazard has been traditionally 

managed by using revetments or other structures to prevent 

bank erosion or to block off floodplain areas that are not 

currently occupied by a river. The degree of this hazard at a 

given location on a floodplain depends on the frequency and 

distance of expected channel movement, which depend on 

the type and elevation of floodplain materials, stream power 

available to transport sediment, and sediment load (Hickin 

and Nanson, 1984; Nanson and Croke, 1992; Kleinhans and 

others, 2012; Konrad, 2012). 

Among mountain rivers in the Puget Sound Basin, the 

probability that the river will occupy a floodplain area is 

associated with the river’s plan form, ranging from floodplains 

with straight channels (lowest probability) to floodplains 

with braided channels (highest probability), which primarily 

reflects increasing slope and, to a lesser extent, decreasing 

streamflow (Beechie and others, 2006). Likewise, in the Puget 

lowland, channel migration rates and occupation of floodplains 

by rivers exhibit a strong contrast between low-gradient 

(<0.001) valleys where rivers have been depositing sediments 

in the Holocene (low migration rates) to steeper (>0.001) 

valleys where rivers have been eroding and reworking valley 

bottom sediments in the Holocene (Collins and Montgomery, 

2011). As a result, river gradient is a primary factor used to 

assess channel occupation hazard on a floodplain.

Sediment load is the second factor that is incorporated 

into the assessment of channel occupation hazard. Generally, 

rivers with high sediment loads are more dynamic, shifting 

their location in response to deposition of sediments along 

their channel. Sediment load is indexed in terms of the fraction 

of the river’s basin that is unregulated with slopes greater 

than 30 degrees. White River is considered unregulated with 

regard to sediment load because Mud Mountain Dam passes 

coarse sediment.

Observed migration rates generally increase with stream 

power (maximum rate of work possible from a river), which 

is the product of gradient, streamflow, and the specific weight 

of water (Hooke, 1980; Nanson and Hickin, 1986; Konrad, 

2012). To account for variation in stream power related to 

streamflow, drainage area is incorporated as a third factor to 

assess this hazard.

A river is more likely to re-occupy locations that it 

recently occupied (Konrad, 2012), which are generally 

located close to the current channel position (Beechie and 

others, 2006, fig. 8; Collins and Montgomery, 2011, fig. 9) 

partly because channel occupation is a consequence of lateral 

migration and widening during floods (Konrad and others, 

2011). To account for the decreasing probability of occupation 

with distance to a river, distance to a river is used as a fourth 

factor to assess this hazard. These factors are combined into 

10 categories representing combinations of river gradient, 

sediment load, and distance to a river scaled by the drainage 

area of the river.

Key assumptions/Uncertainties.—Steep rivers are more 

dynamic across different types and within types. Sediment 

loads are a primary factor contributing to instability, but are 

not incorporated in this hazard. Areas close to rivers have a 

greater probability of channel occupation than areas farther 

away from the river’s current location, but proximity to a 

river will change over time. Revetments intended to protect 

roads, levees, railroads, and other structures may provide some 

protection against, but do not eliminate, the possibility of 

channel occupation. 

Hazard 2a. Bank Erosion

Bank erosion represents a hazard to land use on 

floodplains adjacent to rivers. Bank erosion rates are related 

to many of the same factors as channel occupation, although 

stream power, proximity to a river, bank materials and 

vegetation, bank height, and the curvature of river bends 

are among the most significant ones (Knighton, 1998). As a 

subcategory of channel occupation, bank erosion hazard is 

only evaluated for river bank areas, so the assessment focuses 

on factors related to incidence of bank erosion rather than 

its rate (for example, stream power). Bank erosion is most 

likely for non-cohesive bank material along river segments 

with meander bends. Soils with less than 30 percent silt and 

clay are considered non-cohesive. The assessment of this 

hazard uses sinuosity, which is river path length divided by 

straight-line distance and river gradient to produce six output 

categories. Areas that are not river bank and river bank areas 

formed from consolidated geological materials or cohesive 

sediments are assigned an output value of 0 for this hazard.

Key assumptions/Uncertainties.—Bank erosion 

hazard will change as a river shifts locations on a floodplain. 

Erosion is treated as equally probable along either side of the 

river ignoring local features that affect bank erosion hazard, 

such as meander bends and bluffs. Bank erosion generally 

is more prevalent along the outer banks of meander bends. 

Straight segments have some level of bank erosion hazard, 

but it is not addressed in this assessment. Cohesive strength 

of banks increases with silt and clay content (Knighton, 

1998). Cohesive sediments resist erosion but do not eliminate 

bank erosion hazard. Engineered revetments have not been 

incorporated into this hazard because of the lack of consistent 

and comprehensive mapping. 
Coarse-Resolution Assessment 17

Hazard 2b. Avulsion

Rivers can rapidly shift location, or avulse, when their 

channel becomes blocked by sediment and debris or if 

high flows erode through an obstruction to a flow path that 

is lower than their channel (Slingerland and Smith, 2004; 

Kleinhans and others, 2012). Deposition of sediment and 

debris in reaches where sediment transport capacity decreases 

downstream (for example, transition from a steep, narrow 

valley to a less steep, wider valley, such as where an alluvial 

fan would be expected to form) commonly initiate avulsions. 

Avulsions also can occur when debris or meander bends create 

high flow resistance, backing up water that can access an 

alternative, steeper flow path across the bend or into a former 

channel that is at a lower elevation. Evidence for avulsion 

hazard includes abandoned channels, large in-channel deposits 

of sediment, and an anabranching (multiple channels and 

islands) plan form. Although anabranching rivers indicate 

past avulsion, a high gradient river with high sediment 

and debris loads in unconfined reaches generally can be 

expected to avulse with higher frequency than other types of 

anabranching rivers (Nanson and Knighton, 1996; Kleinhans 

and others, 2012).

Generally, high gradient rivers with high sediment can 

be expected to avulse more frequently than low gradient 

rivers with moderate sediment loads notwithstanding local 

factors that contribute to sediment deposition and affect 

flow direction. Avulsion hazard is assessed for low-lying 

areas along rivers in terms of five categories representing 

combinations of river gradient and sediment load. High 

floodplain areas are assigned an output value of 0 for this 

hazard, although avulsion hazard may be present in some of 

these areas.

Key assumptions/Uncertainties.—Hydraulic gradient 

is a primary driver of sediment transport and deposition 

that drive channel avulsion (Kleinhas and others, 2012) and 

accounts for anabranching channel forms of major rivers in 

the Puget Sound Basin. Additional factors contributing to 

avulsion hazard include large woody debris, local divergence 

of hydraulic gradient (concavity of the river profile), and 

geomorphic features, such as alluvial fans, abandoned 

channels, and meander bends, none of which are included in 

the assessment of this hazard.

Flood-Related Exposures

Exposure 1. Key Facilities

Flooding poses a risk to many facilities located on 

floodplain including those that provide vital services. 

Disruption in water supplies, wastewater treatment, and 

emergency services could be critical issues during floods, 

so exposure of key facilities providing these services is an 

important component of flood-related risks to people in the 

Puget Sound Basin. This type of exposure is assessed by 

assigning a value indicating the type of facility to each cell 

where there is a key facility on a floodplain. All other cells are 

assigned an output value of 0 for the exposure of key facilities.

Key assumptions/Uncertainties.—Facilities are based 

on point source data. The actual footprints of these facilities 

likely extend beyond the 10-m cell assigned to represent the 

facility in the output for this type of exposure. The probability 

of flood-related hazards for these facilities and their 

vulnerability are important considerations in assessing risk but 

are beyond the scope of this assessment.

Exposure 2. Land Uses

People using floodplains are exposed to flood-related 

hazards. The level of exposure of different land uses to 

flood-related hazards varies across the Puget Sound region 

reflecting regional patterns in land use. The output values for 

the assessment of this type of exposure are codes indicating 

the general type of land use on the floodplain. Exposure of 

five types of land uses is assessed—agriculture; transportation/

communication/utility; manufacturing, trade, services; 

residential; and other (cultural/entertainment/recreation; parks, 

mining, timber). 

Key assumptions/Uncertainties.—Exposure is treated 

as uniform within each of the use categories. The location of 

infrastructure and uses of land within parcels will determine 

the actual exposure to flood-related hazards.

Exposure 3. Roads

Floodplains are commonly used as transportation 

corridors. Roads exposed to flooding represent a significant 

risk in the Puget Sound Basin. The exposure of roads depends 

on the connectivity and elevation of floodplain areas adjacent 

to the road. This type of exposure to flooding is assessed in 

terms of the lowest land form (river, low floodplain, high 

floodplain) in the 8-cell neighborhood of any cell with a road.

Key assumptions/Uncertainties.—Road areas were 

based on vector representations of centerlines rather than 

edges. When the roads were gridded, they assume a width of 

1 cell (10 m). This likely underestimates the actual location 

of road areas and their exposure to flood-related hazards. 

River areas and low floodplain are mapped only in areas with 

lidar coverage. 

Coarse-Resolution Assessment

The coarse-resolution assessment summarizes the 

results of the fine-resolution assessment for larger floodplain 

systems and accounts for factors influencing function and 

risk at larger scales than could be addressed in the fineresolution assessment. Function and risk were assessed for 

each of 535 FPUs along major rivers in the Puget Sound 

Basin. FPU boundaries are based on Puget Watershed 

Characterization (PWC) analysis units (Washington State 
18 Geospatial Assessment of Ecological Functions and Flood-Related Risks, Puget Sound Basin, Washington

Department of Ecology, 2013a), which were defined by 

tributary junctions, river gradient, and valley confinement and 

generally are consistent with the river segments used in the 

Salmon and Steelhead Habitat Inventory Program (Washington 

Department of Fish and Wildlife, 2013). In cases where 

PWC analysis units did not extend laterally across a valley 

bottom, they were combined into a single FPU. FPUs range 

from 5 to 15,700 ha in area and encompass segments of rivers 

up to 21 km in length (median segment length of 5.7 km). 

They generally increase in area and length from headwater 

areas where floodplains are confined in narrow valleys to wide 

river deltas where floodplains may be many kilometers wide.

The coarse-resolution assessment provides additional 

FPU-scale attributes and indices that are relevant for assessing 

ecological function and risk. Attributes are measureable 

features, such as floodplain area, hydraulic gradient of the 

river, sinuosity, or upstream regulation (table 4). Indices of 

floodplain function or risk (table 5) are derived to provide 

standardized measures that can be compared among FPUs 

or to integrate multiple factors influencing a function. 

Table 4. Attributes of floodplain units used in the geospatial assessment of ecological functions and flood-related risks on floodplains

along major rivers, Puget Sound Basin, Washington. 

[m, meters; km2, square kilometers]

Attribute [units] Short name

Floodplain unit (FPU) identification number FPU

Drainage area at downstream end of floodplain unit [SQ KM] DrAreaKM2

Length of mainstem river segments in floodplain unit [KM]1 RivLenKM

Water surface gradient [M/M]2 Grad

Downstream change in water surface gradient [M/M]2 DeltaGrad

Ratio of river length to valley length [M/M] Sinuosity

Fraction of drainage area that is unregulated (no large reservoirs) UnregFrac

Fraction of river length that is reservoir ResFrac

Width of river [M] RivWidM

Area of river [SQ KM]3 RivKM2

Area of connected low floodplain [SQ KM]3 FpLowConKM2

Area of connected high floodplain [SQ KM]3 FpHighConKM2

Area of reduced connectivity floodplain [SQ KM] FpRcKM2

Area of valley bottom that is above floodplain or is road, levee, or railroad [SQ KM] NotFpKM2

Fraction of FPU area with lidar data LiDARcov

Area of hillslopes greater than 30 degrees in the drainage area of the FPU [SQ KM] Slope30dKM2

Area of hillslopes with forest cover draining directly to FPU (SQ KM) ForcatchKM2

Area of basin with forest cover ForBasinKM2

Length of mainstem river with bull trout [KM] BTLenKM

Length of mainstem river in FPU with Chinook salmon [KM] ChinLenKM

Length of mainstem river in FPU with steelhead trout [KM] STLenKM

Length of mainstem river in and upstream of FPU with bull trout [KM] BTUpKM

Length of mainstem river in and upstream of FPU with Chinook salmon [KM] ChinUpKM

Length of mainstem river in and upstream of FPU with steelhead trout [KM] STUpKM

Length of non-mainstem river segments (tributaries, anabranches, side channels) OCH_num

Number of non-mainstem river segments (tributaries, anabranches, side channels) OCH_KM

FPU has a mainstem river segment listed as impaired for temperature under Clean Water Act Section 303d [1– yes, 0 –no] TEMP

FPU has a mainstem river segment listed as impaired for dissolved oxygen under Clean Water Act Section 303d 

[1– yes, 0 –no]

DO

FPU has a mainstem river segment listed as impaired for pH under Clean Water Act Section 303d [1– yes, 0 –no] pH

1

Mainstem segments have a drainage area of at least 50 km2, headwater FPUs may have no or very short mainstem river lengths.

2

Values are not available in FPUs with lidar coverage less than 90 percent because river area is not accurate.

3

Gradients are not calculated for FPUs where mainstem length is less than 0.1 km.
Coarse-Resolution Assessment 19

Table 5. Indices of floodplain function and risk used in the geospatial assessment of ecological functions and flood-related risks on 

floodplains along major rivers, Puget Sound Basin, Washington.

Description (units) Short name Function,

hazard, or risk

Floodplain unit FPU

Width of connected floodplain (area/river length) [M] FpConWidM FN1

Width of reduced connectivity floodplain [M] FpRcWidM FN1

Ratio of width of river to square root of drainage area [M/M] RivAR FN1

Ratio of width of connected floodplain to square root of drainage area [M/M] FpConAR FN1

Ratio of width of reduced connectivity floodplain to square root of drainage area [M/M] FpRcAR FN1

Conveyance index: product of river gradient and river width [M] Convey FN1, HAZ1

Difference in conveyance index of FPU and sum of conveyance index for upstream, bordering 

floodplain units

DeltaConvey FN1, FN2, HAZ1, HAZ2

Ratio of width of connected, undeveloped floodplain to square root of drainage area FpConUndAR FN2

Ratio of width of reduced connectivity, undeveloped floodplain to square root of drainage area FpRcUndAR FN2

Ratio of width of connected, developed floodplain to square root of drainage area FpConDevAR FN2, HAZ1, EXP2

Fraction of river margin with forest cover BankForFrc FN2

Fraction of river margin undeveloped, not forest cover BankUndFrc FN2

Fraction of river margin developed BankDevFrac FN2, HAZ2, EXP2

Fraction of river margin with roads, levees, or railroads BankRLRFrac FN2, HAZ2, EXP3

Floodplain width with wetland, hydric soil, or shallow groundwater; silt/clay >30 percent, 

organic matter >2 percent [M]

FpWetScOmW FN3

Floodplain width with wetland, hydric soil, or shallow groundwater; silt/clay >30 percent, 

organic matter <2 percent [M]

FpWetScW FN3

Floodplain width with wetland, hydric soil, or shallow groundwater; silt/clay <30 percent, 

organic matter <2 percent [M]

FpWetW FN3

Connected, forested floodplain width [M] FpConForW FN4

Connected, undeveloped but not forested floodplain width [M] FpConUndW FN4

Reduced connectivity, forested floodplain width [M] FpRcForW FN4

Reduced connectivity, undeveloped, not forested floodplain width [M] FpRcUndW FN4

Fraction of areas draining directly to floodplain unit (not upstream units) that is forested CatchForFrc FN4

Areas of undeveloped river edge divided by mainstem river length RivEdgeUndW FN5

Connected floodplain width divided by river width Confine FN5

Difference between confinement and potential confinement (combined width of connected 

and reduced connectivity floodplain/river width) up to a potential confinement of 4 in 

FPUs with confinement less than 4 [M/M]

AddConfine FN5

In cases where a function or risk is represented by an 

area‐based measure (for example, the area of connected, 

forested floodplain in an FPU), the measure is divided by 

mainstem river length to produce a mean width of floodplain 

in the FPU with the function or risk. In cases where a function 

or risk depends on the interaction of factors, those factors are 

combined into an index (for example, river width and gradient 

are combined into an index of flood conveyance). Tables 

with the values of the attributes and indices for each FPU are 

available at http://pubs.usgs.gov/sir/2015/5033/ as part of this 

report (fpu_attribute.csv and fpu_indices.csv).

The relevance of the selected attributes and indices 

to function and risk is described briefly for context. The 

assessment does not provide an exhaustive set of attributes 

and indices that would be used in either a comprehensive or a 

targeted evaluation of ecological functions and risks to people 

on floodplains. Indices that integrate many factors influencing 

function or risk over large areas (for example, Hauer and 

Smith, 1998) were not derived as part of the coarse-scale 

assessment because of compounding uncertainty from source 

data, weighting of multiple factors, and spatial scaling.
20 Geospatial Assessment of Ecological Functions and Flood-Related Risks, Puget Sound Basin, Washington

Floodplain Functions

Function 1. Store and Convey Floods

Floodplain storage and conveyance depend on the river 

and floodplain area available for flooding, the hydraulic 

gradient of the river during a flood, and the depth of water 

during a flood. The area of river, connected low floodplain, 

and connected high floodplain indicate the capacity available 

to store and convey flood waters in each FPU (table 

fpu_attributes.csv, variables RivKM2, FpLowCnKM2, 

and FpHighCnKM2, respectively). The area of reduced 

connectivity floodplain (table fpu_attributes.csv, variable 

FpRcKM2) indicates the potential to increase storage and 

conveyance through floodplain reconnection projects. The 

areas of rivers and floodplains are standardized by the length 

of mainstem river in each FPU to produce the mean widths 

of river, connected floodplain, and reduced connectivity 

floodplain for each FPU (table fpu_indices.csv, variables 

RivWidM, FpConWidM, FpRcWidM). 

River and floodplains widths may be more useful than 

areas for comparisons of conditions across FPUs, but FPUs 

with higher streamflow can be expected to have wider rivers 

and floodplains. The non-dimensional ratios of river or 

floodplain width to the square root of drainage area (table fpu_

indices.csv, variables RivAR, FPConAR, FpRcAR) provide an 

index of the area of river or floodplain in an FPU that accounts 

for differences in the length of river and streamflow among 

FPUs. Width-area ratios can be used to identify FPUs that 

have relatively narrow or wide rivers or floodplains.

Function 2. Regulate Sediment and Wood 

Supplies in River Networks

The availability of sediments and coarse organic 

debris for transport depends on upstream sources of these 

materials and longitudinal connectivity between the source 

area and a floodplain. Dams disconnect source areas from 

downstream floodplains (table fpu_attributes.csv, variable 

UnregFrac). Areas with steep slopes, defined as greater than 

30 degrees, in the unregulated portion of the drainage area of 

an FPU (table fpu_attributes.csv, variable Slop30dUpKM2) 

disproportionately contribute sediment to river networks. 

Steep, forested areas in unregulated portions of a river 

basin indicate the overall availability of wood supplies to a 

floodplain (table fpu_attributes.csv, variable ForCatchKM2. 

Land cover on river banks can affect the delivery of sediment 

and wood to rivers and may indicate the presence of bank 

protection structures that limit bank erosion. Land cover on 

banks is expressed in terms of the fraction of river bank cells 

with forest, undeveloped cover, developed cover, or with 

roads/levees/railroads (fpu_indices.csv, variables BankForFrc, 

BankUndFrc, BankDevFrc, BankRLRFrc).

Function 3. Retain and Transform Nutrients and 

Contaminants

Retention and transformation of water pollutants on 

floodplains require sources of pollutants, areas where the 

pollutants are stored in surface water, on the land surface, or 

sorbed to sediments. Retention of particulates and compounds 

sorbed to particulates is most likely in low gradient valley 

segments (table fpu_attributes.csv, variable Grad). The area 

of floodplain with indicators of anaerobic conditions, finegrained sediments, and high organic matter as defined in 

the fine-scale assessment may indicate the capacity of the 

floodplain to retain and transform nutrients and contaminants. 

The areas in each FPU with these factors were divided by the 

length of mainstem river in each FPU to produce the width of 

floodplains with anaerobic indicators (table fpu_indices.csv, 

variable FpWetW), with anaerobic indicators and fine-grained 

sediments (variable FpWetScW), and these factors plus high 

organic matter (variable FpWetScOmW).

Function 4. Support Forest Ecosystems

The extent of connected and reduced connectivity 

floodplain forests are expressed as mean widths for each 

FPU (table fpu_indices.csv, variables FpConForW and 

FpRcForW, respectively). Undeveloped areas that could 

potentially support forest on connected floodplains and 

reduced connectivity floodplains also are expressed in terms 

of mean widths for each FPU (table fpu_indices.csv, variables 

FpConUndW and FpRcUndW). Floodplain forests can be part 

of larger forest ecosystems that extend across valleys, serving 

as migration corridors and ecotones between terrestrial and 

aquatic habitats. Given the intensity of land use on floodplains, 

fragmentation of floodplain forests may be a significant source 

of fragmentation of larger forest ecosystems. As a result, the 

fraction of forest cover on hillslopes draining directly to an 

FPU, but not including areas that drain to upstream FPUs, 

(table fpu_indices.csv, variable CatchForFrc) provides a 

measure of the potential value of an FPU in maintaining 

connectivity across a larger forest ecosystem.

Function 5. Provide Aquatic Habitats

Floodplain-river systems are a mosaic of different 

habitats distinguished by their connectivity to a river in time 

and space and the seral stage of vegetation on the floodplain 

(Ward and others, 2002; Konrad and others, 2011). In the 

Pacific Northwest, the diversity of aquatic habitats on a 

floodplain and the presence of complex habitat types are 

associated with reach-scale plan form of river channels 

(straight, meandering, anastomosing, island-braided, braided), 

which depends in part on valley gradient and confinement 

(Beechie and others, 2006). 
Overview of Assessment Outputs 21

Confinement is the ratio of valley or floodplain width 

to river width (Knighton, 1998). When a floodplain is about 

twice as wide as a river channel (confinement greater than 3), 

the river is largely unconfined by its valley and displays 

functional differences including greater retention of organic 

material and greater biodiversity (Bellmore and Baxter, 2013). 

Unconfined rivers can inundate large areas during and after 

floods that provide aquatic habitats, migrate to new locations 

and create off-channel aquatic habitats at the former location 

of the channel, and support complex food webs that include 

anadromous salmonids (Beechie and others, 2006; Konrad 

and others, 2008; Konrad, 2012; Bellmore and others, 2013). 

Rivers with confinement less than 3 typically have a single 

channel with a limited number and area of off-channel aquatic 

habitats (Bellmore and Baxter, 2013). Rivers can be confined 

with little or no floodplain because: their valley is relatively 

narrow; they are incised into valley fill deposits and have not 

developed a floodplain since incision; or they are disconnected 

from their floodplain by roads, railroads, levees, or other 

structures. The potential to reduce anthropogenic confinement 

of a river can be estimated as the difference between the 

confinement calculated using connected floodplain width and 

confinement calculated using connected and disconnected 

floodplain width (table fpu_indices.csv, variable AddConfine). 

Although the current and potential complexity of aquatic 

habitats is likely to be related to confinement, the length 

of mainstem channels in an FPU provide an index of its 

complexity incorporating both meander bends (which indicate 

pool-riffle morphology) and islands, which provide additional 

edge habitats. The length of mainstem channel divided by its 

straight-line distance, or sinuosity, (table fpu_attributes.csv, 

variable Sinuosity) provides an index of mainstem habitat 

complexity that can be compared among FPUs.

Upstream sources of sediment and wood contribute to the 

development of complex aquatic habitats (table fpu_attributes.

csv, variables Slp30degKM2, ForCatchKM2, ForBasinKM2). 

In addition to mainstem river segments, tributaries, 

anabranches, and side channels (table fpu_attributes.csv, 

OCH_num and OCH_KM) represent other significant large 

habitat features in river networks (Beechie and others, 

2006; Rice and others, 2006). River edges (fpu_indices.csv, 

variable RivEdgeUndW) are particularly important rearing 

areas for juvenile salmonids because of shallow, low velocity 

streamflow (Beechie and others, 2005).

The significance of floodplains to salmonids depends 

in part on production of juveniles in the river network. The 

total length of stream in an FPU and upstream of an FPU with 

bull trout, steelhead trout, and Chinook salmon (table fpu_

attributes.csv, variables BTLenKM, BTUpKM, STLenKM, 

STUpKM, and ChinLenKM, ChinUpKM) serves as a gross 

index to represent variation across FPUs in the number of 

juvenile salmonids that could use floodplain habitats.

Salmonid production can be limited by water quality 

in rivers or streams draining to Puget Sound (Marshall and 

others, 2014). FPUs with river segments listed as impaired 

for temperature, dissolved oxygen, or pH (Washington State 

Department of Ecology, 2012) (table_fpu_attributes.csv, 

variables TEMP, DO, pH) may have reduced capability to 

support salmonids.

Flood-Related Hazards

Flood inundation hazard is most likely in low-lying, low 

gradient areas (table fpu_attribute, variables FpLowCnKM2 

and Grad) along rivers with a large, unregulated fraction of 

their drainage area (table fpu_attribute, variable UnregFrc). 

Channel occupation (hazard 2) is associated with highgradient rivers (table fpu_attribute, variable Grad), high 

sediment loads (table fpu_attribute, variable Slope30dKM2), 

and a downstream reduction in gradient (table fpu_attributes.

csv, variable DeltaGrad) or conveyance (table fpu_indices.

csv, variable DeltaConvey). Floodplain units where DeltaGrad 

or DeltaConvey are less than 0 indicate a decreasing 

capacity of the river to convey floods and the potential for 

higher stages, sediment deposition in these reaches, and 

an increasing probability of channel avulsion (Collins and 

Montgomery, 2011). 

Flood-Related Exposure

The ratio of connected, developed floodplain width to the 

square root of drainage area (table fpu.indices.csv, variable 

FpConDevAR) provides a general index of exposure of 

residential, agricultural, and other commercial development 

to flooding.

Overview of Assessment Outputs

The Puget Sound Basin has extensive river-floodplain 

systems in its major valleys. These systems cover about 

1,600 km2 including rivers (about one-fourth of system 

area), low floodplains (about three-eighths of system area) 

and three‐eighths high floodplains (about three-eighths of 

system area) (table 4). Large river deltas, which can be many 

kilometers wide, comprise most of the floodplain area. The 

FPU median floodplain width is 222 m.

Floodplain connectivity indicates a broad range of 

ecological functions including the current and potential 

capacity of floodplains to store and convey floods, transport 

sediment and wood, retain and transform water pollutants, 

support forest ecosystems, and provide habitats for aquatic 

species including endangered salmonids. Almost one-half 

of all floodplain areas (743 km2) in the Puget Sound have 

been disconnected to some extent from their rivers by roads, 

levees, and railroads (fig. 3), but disconnection is concentrated 

in lowland areas where floodplains generally are wider. 
22 Geospatial Assessment of Ecological Functions and Flood-Related Risks, Puget Sound Basin, Washington

As a consequence, disconnection is much less prevalent in 

terms of the percent of river length than percent of floodplain 

area. Indeed, the FPU median width of connect floodplain is 

199 m (table floodplain_indices, variable FpConWidM) and 

174 of 535 FPUs have no disconnection of floodplain areas. 

Regulation of rivers by dams also affects floodplain 

connectivity in the Puget Sound Basin—10 percent of 

floodplains units, 10 percent of major river length (281 km), 

and 7 percent of floodplain area (108 km2) have more than 

50 percent of their drainage area regulated. The effect of 

regulation, however, depends on dam operations and reservoir 

capacity and is beyond the scope of this assessment. 

Most (1,084 of 1,603 km2) of the floodplains delineated 

in for this assessment are considered “Special Flood Hazard 

Areas” with an annual probability of flooding of greater 

than 1 percent per year. Floodplain areas outside of the 

SFHA (521 km2) are in rural areas where SFHAs have not 

been delineated, are protected from flooding by levees, or 

otherwise have an annual probability of flooding of less than 

0.01. SFHAs include an additional 264 km2 outside of the 

floodplains delineated for this assessment, most of which 

is lake or reservoir. The lower Stillaguamish River valley 

and Snoqualmie River valley downstream of the Tolt River 

and around Falls City have the largest SFHAs that are not 

delineated as floodplains in this assessment. In these river 

segments, the increase in stage during large floods may be 

greater than estimated from the regional relation (fig. 2). 

Development on 196 km2of connected floodplains represents 

risks to many people in the region who rely on connected 

floodplain for agriculture (176 km2) and who live, work, 

or travel in connected floodplains with medium and high 

development (14 km2) (table 6).

Anthropogenic confinement of rivers indicates where 

floodplain reconnection could improve ecological function and 

reduce flood risk. The ratio of connected floodplain width to 

the square root of drainage area (variable FpConAR) provides 

a metric of the extent of floodplains while accounting for river 

size. The median value for floodplains in the Puget Sound 

Basin is 0.01, which represents a 100-m wide connected 

floodplain for a river that has a drainage area of 100 km2 

and a 300-m wide connected floodplain for a river that has a 

drainage area of 900 km2 (fig. 4).

watac14-0968_fig 03

Connected, undeveloped river

and low floodplain (380 km2

)

Connected, undeveloped

high floodplain (268 km2

)

Connected, developed river

and floodplain (202 km2

)

Reduced connectivity,

undeveloped river and

floodplain (158 km2

)

Reduced connectivity, developed

floodplain (602 km2

)

Figure 3. Connectivity and development of floodplains in Puget Sound Basin, 

Washington. Development includes impervious, developed non-impervious, and 

agricultural areas.
Overview of Assessment Outputs 23

100

1,000

Drainage area, in square kilometers

Connected floodplain width, in meters

10

10

100 1,000 10,000

Ratio of width of connected floodplain, in kilometers,

 to square root of drainage area, in square kilometers, 

 FpConAR = 0.01

Figure 4. Width of connected floodplain area as a function of drainage area for 

mainstem rivers in the Puget Sound Basin, Washington. Line represents the median ratio 

of floodplain width to the square root of drainage area (W = 10 Area0.5) for connected 

floodplains in the Puget Sound Basin. Relatively wide connected floodplains plot above 

the line, relatively narrow connected floodplains plot below the line.

Table 6. Summary of floodplain connectivity and land use resulting from the geospatial assessment 

of ecological functions and flood-related risks on floodplains along major rivers, Puget Sound Basin, 

Washington.

[Land use based on National Land Cover Database 2011 land cover categories (National Oceanic and Atmospheric 

Administration, 2014). Agricultural use based on tax parcel designation (Washington State Department of Ecology, 2012)]

Flood plain connectivity

Land use, in square kilometers

Total area Undeveloped

area

Area of medium/

high development

Agricultural 

area

Connected floodplain including river areas 850 648 14 176

Connected high floodplain 406 268 11 109

Connected low floodplain and river 444 380 4 67

Connected low floodplain 252 200 2 53

Connected river 192 180 1 14

Reduced connectivity floodplain 760 158 89 406

Total floodplain and river 1,610 806 103 581
24 Geospatial Assessment of Ecological Functions and Flood-Related Risks, Puget Sound Basin, Washington

Ecological functions of connected floodplains that 

are at least three times the river width are distinct from 

narrower floodplains (see Bellmore and Baxter, 2013). 

Although some river valleys are too narrow to allow the 

development of wide floodplains, about one-half by length 

of the major rivers in the Puget Sound Basin are in valleys 

where floodplains (connected and reduced connectivity) 

are more than three times as wide as their channel (fig. 5). 

Roads, railroads, and levees have confined about 760 km 

of these rivers to connected floodplains less than three 

times as wide as their channels. The loss of unconfined 

rivers is of particular ecological significance because they 

support complex habitats including islands, side channels, 

wetlands, and provide shallower, low velocity edge habitat 

than confined rivers (Beechie and others, 2006; Konrad and 

others, 2008). 

In addition to confining rivers, roads, levees and 

railroad have fragmented floodplains into smaller areas that 

limit the flow of water, transport of materials, and migration 

of organisms across floodplains. Prior to construction of 

this infrastructure, most floodplains in Puget Sound were 

in contiguous areas of at least 100 km2 (fig. 6). Currently, 

most floodplains are in fragments less than 10 km2.

Less than 0.1 0.1–1.0 1–10 10–100 100–1,000

Fragment size, in square kilometers

Cumulative floodplain area, in square kilometers

0

200

400

600

800

1,000

1,200

Floodplains fragmented by roads, levees, and railroads in addition 

to canyons and other topographic features

Floodplains fragmented by canyons or other topographic features

EXPLANATION

Figure 6. Cumulative floodplain area for fragment sizes, Puget Sound Basin, Washington.

watac14-0968_fig05

Less than 3 More than 3

Confinement ratio: floodplainwidth / river width

River length, in kilometers

0

500

1,000

1,500

2,000

Confinement calculated only using the width of connected flood plains, 

 excluding areas separated from rivers by roads, railroads, or levees

Confinement calculated using the width of all floodplains

EXPLANATION

Figure 5. Length of confined and unconfined mainstem 

rivers in the Puget Sound Basin, Washington.
References Cited 25

Summary

Ecological functions and flood-related risks were 

assessed for areas along the 17 major rivers in the Puget 

Sound Basin. For the fine-resolution assessment, available 

source data and simple GIS models were used to derive 

categories of function and risk as raster outputs with a 

10-meter resolution. These categories can be used to identify 

likely factors that would have to be addressed to recover 

floodplain function or reduce flood risk at a given location. In 

general, connectivity and land cover/land use are the primary 

attributes that influence ecological function and risk to people 

on floodplains. Both connectivity and land cover/land use 

would have to be addressed to improve function and reduce 

risk on most floodplains.

Attributes influencing floodplain function and risk 

were summarized for larger-scale floodplain units in 

the coarse‐resolution assessment. The coarse-resolution 

assessment also includes indices that have been standardized 

to permit comparison between floodplain units of different 

areas and drainage areas. The attributes and indices from the 

coarse-resolution assessment can be combined to identify 

areas where ecological function can be improved and risks to 

people can be reduced. This is one step in the development of 

feasible objectives for integrated floodplain management for 

the Puget Sound region. 

The assessment outputs are available as geospatial 

datasets in Konrad (2015; http://dx.doi.org/10.5066/

F7DR2SJC). The uncertainty in the outputs is determined 

by source data and the steps used to derive the datasets. All 

outputs have uncertainty around the boundaries of floodplains, 

which are based on spatial extrapolation of the approximate 

water- surface elevation of rivers and assumptions about 

the increase in those elevations during floods. In general, 

functions that integrate many factors, such as transforming 

nutrients and contaminants or providing aquatic habitats, have 

greater uncertainty. These functions also are affected by finescale heterogeneity in those factors, which introduces more 

uncertainty in the results. 

In many cases, a more general assessment of floodplain 

function and risk using less derived information, such as 

connectivity and land cover, may be sufficient and more 

reliable for planning and management of floodplains. More 

than one-half of the floodplains along major rivers in the 

Puget Sound Basin have been disconnected to some extent 

by roads, levees, and railroads or have been developed for 

agricultural, other commercial, residential, or transportation 

uses. Ecological functions are likely impaired on these 

floodplains and there are likely to be flood-related risks to the 

people using these floodplains. In general, the most feasible 

opportunities to improve floodplain function may be in areas 

that either have been disconnected or developed, but not both. 

Likewise, the greatest opportunities to reduce risk may be in 

connected floodplain areas with development.

Acknowledgments

The U.S. Geological Survey received funding 

for this assessment from The Nature Conservancy 

through the Washington State Department of Ecology 

and U.S. Environmental Protection Agency’s National 

Estuary Program. Theresa Olsen, U.S. Geological Survey, 

assisted with compiling and analyzing geospatial data. 

This assessment incorporates contributions from many 

people including: Allison Bailey, Sound GIS, Jack Barbash, 

U.S. Geological Survey; Tim Beechie, National Oceanic and 

Atmospheric Administration; Bob Black, U.S. Geological 

Survey; Derek Booth, Stillwater Sciences; Scott Campbell, 

U.S. Army Corps of Engineers; Bob Carey, The Nature 

Conservancy; Brian Collins, University of Washington; 

Paula Cooper, Whatcom Co.; Steve Cox, U.S. Geological 

Survey; Paul Cereghino, National Oceanic and Atmospheric 

Administration; Dennis Dixon, Pierce Co.; Susan Grigsby, 

Washington State Department of Ecology; Polly Hicks, 

National Oceanic and Atmospheric Administration; 

Hans Hunger, Pierce Co.; Colin Hume, Washington 

State Department of Ecology; Kris Johnson, The Nature 

Conservancy; Nick Legg, Washington State Department of 

Ecology; Rand Little, Seattle Public Utilities; Julie Morse, The 

Nature Conservancy; Mark Munn, U.S. Geological Survey; 

Patricia Olson, Washington State Department of Ecology; 

Lorin Reinhelt, King County; Kelly Slattery, Washington State 

Department of Ecology; Stephen Stanley, Washington State 

Department of Ecology; Kelly Stone, Federal Emergency 

Management Agency; David St. John, Puget Sound 

Partnership; Jim Tesoriero, U.S. Geological Survey; and 

Christian Torgersen, U.S. Geological Survey.

References Cited

Baldwin, D.S., and Mitchell, A.M., 2000, The effects of 

drying and re-flooding on the sediment and soil nutrient 

dynamics of lowland river-floodplain systems—A synthesis: 

Regulated Rivers—Research and Management, v. 16, no. 5, 

p. 457–467.

Beechie, T.J., Liermann, M., Beamer, E.M., and 

Henderson, R., 2005, A classification of habitat types in a 

large river and their use by juvenile salmonids: Transactions 

of the American Fisheries Society, v. 134, p. 717–729.

Beechie, T.J., Liermann, M., Pollock, M.M., Baker, S., 

and Davies, J., 2006, Channel pattern and riverfloodplain dynamics in forested mountain river systems: 

Geomorphology, v. 78, p. 124–141.
26 Geospatial Assessment of Ecological Functions and Flood-Related Risks, Puget Sound Basin, Washington

Bellmore, J.R., and Baxter, C.V., 2013, Effects of geomorphic 

process domains on river ecosystems: a comparison of 

floodplain and confined valley segments, River Research 

and Applications, v. 30, p. 617–630.

Bellmore, J.R., Baxter, C.V., Martens, K., and Connolly, P.J., 

2013, The floodplain food web mosaic—A study of its 

importance to salmon and steelhead with implications for 

their recovery: Ecological Applications, v. 23, p. 189–207.

Chow, V.T., 1959, Open-channel hydraulics: New York, 

McGraw-Hill, 680 p.

Collins B.D., and Montgomery, D.R., 2011, The legacy of 

Pleistocene glaciation and the organization of lowland 

alluvial process domains in the Puget Sound region: 

Geomorphology, v. 126, p. 174–185.

Environmental Systems Research Institute, 2013, ArcGIS 

Desktop 10 Service Pack 5.

Federal Emergency Management Agency, 2013, Flood map 

service center, accessed January 27, 2013, at https://msc.

fema.gov/portal.

Fetherston, K.L, Naiman, R.J., and Bilby, R.E., 1995, 

Large woody debris, physical process, and riparian forest 

development in montane river networks of the Pacific 

Northwest: Geomorphology, v. 13, p. 133–144.

Hauer, F.R, and Smith, R.D., 1998, The hydrogeomorphic 

approach to functional assessment of riparian wetlands—

Evaluating impacts and mitigation on river floodplains in 

the U.S.A.: Freshwater Biology, v. 40, p. 517–530.

Hickin, E.J., and Nanson, G.C., 1984, Lateral migration rates 

of river bends: Journal of Hydraulic Engineering, v. 110, 

p. 1,557–1,567. 

Hooke, J.M., 1980, Magnitude and distribution of rates 

of river bank erosion: Earth Surface Processes, v. 5, 

p. 143‐157.

Jones, J., 2006, Side channel mapping and fish habitat 

suitability analysis using Lidar topography and 

orthophotography: Photogrammetric Engineering and 

Remote Sensing, v. 72, no. 11, p. 1,202–1,206.

Kleinhans, M.G., de Hhas, T., Lvooi, E., and Makaske, B., 

2012, Evaluating competing hypotheses for the origins and 

dynamics of river anastomosis: Earth Surface Processes and 

Landforms, v. 37, p. 1,337–1,351.

Knighton, D., 1998, Fluvial forms and processes: London, 

Arnold Publishers, 383 p.

Konrad C.P., 2012, Reoccupation of floodplains by rivers and 

its relation to the age structure of floodplain vegetation: 

Journal of Geophysical Research, v. 117, no. G00N13.

Konrad, C.P., 2015, Geospatial data on ecological functions 

and flood-related risks to people on major river floodplains 

in the Puget Sound Basin, Washington: U.S. Geological 

Survey data release, http://dx.doi.org/10.5066/F7DR2SJC.

Konrad, C., Berge, H., Fuerstenberg, R., Steff, K., Olsen, T., 

and Guyenet, J., 2011, Channel dynamics in the Middle 

Green River, Washington, from 1936–2002: Northwest 

Science, v. 85, p. 1–14.

Konrad, C.P., Black, R.W., Voss, F., and Neale, C.M.U., 

2008, Integrating remotely acquired and field data to assess 

effects of setback levees on riparian and aquatic habitats in 

glacial‐melt water rivers: River Research and Applications, 

v. 24, no. 4, p. 355–372.

Latterell, J.J., Bechtold, J.S., O’Keefe, T.C., van Pelt, R., and 

Naiman, R.J., 2006, Dynamic patch mosaics and channel 

movement in an unconfined river valley of the Olympic 

Mountains: Freshwater Biology, v. 51, p. 523–544.

Marcus, W.A., 1989, Regulating contaminated sediments 

in aquatic environments—A hydrologic perspective: 

Environmental Management, v. 13, p. 703–713.

Marshall, R., Era-Miller, B, and Collyard, S., 2014, Integrated 

ambient monitoring follow-up study in Indian Creek: an 

investigation of the causes of biological impairment and a 

further demonstration of the instream monitoring approach: 

Washington State Department of Ecology, Publication 

No. 14-03-050.

Naiman, R.J., Bechtold, J.S., Beechie, T.J., Latterell, J.J., 

and van Pelt, R., 2010, A process-based view of floodplain 

forest pattern in coastal river valleys of the Pacific 

Northwest: Ecosystems, v. 13, p. 1–31.

Nanson, G.C., and Croke, J.C., 1992, A genetic classification 

of floodplains: Geomorphology, v. 4, p. 459–486.

Nanson, G.C., and Hickin, E.J., 1986, A statistical examination 

of bank erosion and channel migration in western Canada: 

Bulletin of the Geological Society of America, v. 97, 

p. 497–504.

Nanson, G.C., and Knighton, A.D., 1996, Anabranching rivers: 

their cause, character, and classification: Earth Surface 

Processes and Landforms, v. 21, p. 217–239.

National Oceanic and Atmospheric Administration, 2014, 

2010 Coastal change analysis program regional land cover 

Washington, accessed April 25, 2014, at http://www.csc.

noaa.gov/htdata/ccap/ccap_regional_dates/wa_2011_ccap_

land_cover.zip.

National Research Council, 1992, Restoration of aquatic 

ecosystems—Science, technology, and public policy, water 

science and technology board: Washington D.C., National 

Academy Press, 552 p.
References Cited 27

National Resource Conservation Service, 2013, SSURGO 

Database, accessed March 11, 2014, at http://websoilsurvey.

sc.egov.usda.gov/App/HomePage.htm. 

Olson, P.L., Legg, N.T., Abbe, T.B., Reinhart, Mary Ann, 

and Radloff, J.K., 2014, A methodology for delineating 

planning-level channel migration zones: Olympia, 

Washington, Shorelands and Environmental Assistance 

Program, Washington State Department of Ecology, 

Publication No. 14-06-025, 83 p. 

Orr, C.H., Stanley, E.H., Wilson, K.A., and Finlay, J.C., 2007, 

Effects of restoration and reflooding on soil denitrification 

in a leveed midwestern floodplain: Ecological Applications, 

v. 17, p. 2,365–2376, http://dx.doi.org/10.1890/06-2113.1

Pinay, G., Black, V.J., Planty-Tabacchi, A.M., Gumiero, 

B., and Decamps, H., 2000, Geomorphic control 

on denitrification in large river floodplain soils: 

Biogeochemistry, v. 50, p. 163–182.

Pinay, G., Clement, J.C., and Naiman, R.J., 2002, Basic 

principles and ecological consequences of changing 

water regimes on nitrogen cycling in fluvial systems: 

Environmental Management, v. 30, p. 481–491.

Puckett, L.J., Zamora, C., Essaid, H., Wilson, J.T., Johnson, 

H.M., Brayton, M.J., and Vogel, J.R., 2008, Transport and 

fate of nitrate at the ground-water-surface-water interface: 

Journal of Environmental Quality, v. 37, p. 1,034–1,050.

Puget Sound Lidar Consortium, 2011, Puget Sound Lidar 

supermosaic, http://pugetsoundlidar.ess.washington.edu/.

Rapp, C., and Abbe, T., 2003, A framework for delineating 

channel migration zones: Washington State Department of 

Ecology Report 03-06-027, 135 p. 

Rice, S.P., Ferguson, R.I., and Hoey, T.B., 2006, Tributary 

control of physical heterogeneity and biological diversity 

at river confluences: Canadian Journal of Fisheries and 

Aquatic Sciences, v. 63, p. 2,553–2,566.

Slingerland, R., and Smith, N.D., 2004, River avulsions and 

their deposits: Annual Reviews of Earth and Planetary 

Science, v. 32, p. 257–285.

Smith, J.A., Wikowski, P.J., and Fusillo, T.V., 1988, Manmade 

organic compounds in the surface waters of the United 

States—A review of current understanding: U.S. Geological 

Survey Circular 1007, 104 p.

Tesoriero, A.J., and Puckett, L.J., 2011, O2 reduction 

and denitrification rates in shallow aquifers: Water 

Resources Research, v. 47, no. W12522, 17 p., 

doi:10.1029/2011WR010471.

U.S. Army Corps of Engineers, 2013, National levee database, 

accessed March 11, 2014, at http://nld.usace.army.mil/

egis/f?p=471:1:.

U.S. Army Corps of Engineers, 2014, Skagit River flood 

risk management general investigation, Skagit County, 

Washington—Draft feasibility report and environmental 

impact statement—Appendix B: Hydraulics and Hydrology, 

478 p., http://www.nws.usace.army.mil/Portals/27/docs/

civilworks/projects/Skagit%20GI/SkagitGI-AppB-HHMAY2014.pdf.

U.S. Census Bureau, 2010, Washington primary and secondary 

roads, accessed September 24, 2014, at http://geography.

wa.gov/data-products-services/data/data-catalog.

U.S. Geological Survey, 2012, National elevation dataset, 

10 m, accessed on March 11, 2014, at http://ned.usgs.gov/.

U.S. Geological Survey, 2013, National hydrography dataset 

high resolution hydrography, accessed April 25, 2014, at 

ftp://nhdftp.usgs.gov/DataSets/Staged/States/FileGDB/

HighResolution/NHDH_WA_931v210.zip.

U.S. Geological Survey, 2014, National land cover database 

2011 percent developed imperviousness, http://www.mrlc.

gov/nlcd11_data.php.

Ward, J.V., and Stanford, J.A., 1995, The serial discontinuity 

concept—Extending the model to floodplain rivers: 

Regulated Rivers—Research and Management, v. 10, 

p. 159–168.

Ward, J.V., Tockner, K., Arscott, D.B., and Claret, C., 2002, 

Riverine landscape diversity: Freshwater Biology, v. 47, 

p. 517–539.

Washington Department of Fish and Wildlife, 2013, 

Distributions of Chinook salmon, steelhead trout, and bull 

trout, accessed March 11, 2014, at http://apps.wdfw.wa.gov/

salmonscape/map.html.

Washington Department of Health, 2013, Water systems, 

septic systems, fire stations, hospitals, emergency medical 

services, accessed September 24, 2014, at http://geography.

wa.gov/data-products-services/data/data-catalog.

Washington Department of Natural Resources, 2010, Surface 

geology 1:100,000, accessed September 24, 2014, at http://

geography.wa.gov/data-products-services/data/data-catalog.

Washington Department of Transportation, 2013, Railroads 

1:24,000, accessed September 24, 2014, at http://geography.

wa.gov/data-products-services/data/data-catalog.

Washington State Department of Ecology, 2010a, Statewide 

levee inventory and flood protection study—Report 

on certification and accreditation: Bellingham, Wash., 

Western Washington University, prepared by the Resilience 

Institute, publication number 10-06-029, 123 p., accessed 

September 24, 2014, at https://fortress.wa.gov/ecy/

publications/publications/1006029.pdf.
28 Geospatial Assessment of Ecological Functions and Flood-Related Risks, Puget Sound Basin, Washington

Washington State Department of Ecology, 2010b, Washington 

state land use, accessed May 17, 2014, at http://www.ecy.

wa.gov/services/gis/data/planningCadastre/landuse.htm.

Washington State Department of Ecology, 2012, Washington 

water quality assessment 303(d), accessed May 17, 2014, at 

http://www.ecy.wa.gov/services/gis/data/data.htm. 

Washington State Department of Ecology, 2013a, Puget 

Sound watershed characterization, accessed May 17, 2014, 

at http://www.ecy.wa.gov/services/gis/data/pugetsound/

characterization.htm.

Washington State Department of Ecology, 2013b, Permit and 

reporting information system, accessed April 25, 2014, at 

http://www.ecy.wa.gov/PROgrams/wq/permits/paris/index.

html.

Wolman, M.G., and Leopold, L.B., 1957, River flood plains—

Some observations on their formation: U.S. Geological 

Survey Professional Paper 282-C, p. 87–109.
Publishing support provided by the U.S. Geological Survey

Science Publishing Network, Tacoma Publishing Service Center

For more information concerning the research in this report, contact the

 Director, Washington Water Science Center

U.S. Geological Survey

934 Broadway, Suite 300

Tacoma, Washington 98402 

http://wa.water.usgs.gov
Konrad—Geospatial Assessment of Ecological Functions and Flood-Related Risks, Puget Sound Basin, Washington—Scientific Investigations Report 2015–5033

ISSN 2328-0328 (online)

http://dx.doi.org/10.3133/sir20155033