Supplementary Material 1 – Precipitation and Discharge in the Study Area
Our study evaluates and validates methods for determining streamflow duration classes in the
Pacific Northwest. Streamflow duration classes – perennial, intermittent, or ephemeral –
categorize how long streamflow lasts over a year (Fig. SM-1). Perennial streams flow yearround during a typical year, receiving appreciable quantities of water from numerous sources but
with consistent groundwater inputs required throughout the year (Winter et al. 1998; Winter
2007). In cases where groundwater aquifers are unable to supply sufficient quantities of water,
intermittent streams cease to flow during dry periods (Mosley and McKerchar 1993; Rains and
Mount 2002; Rains et al. 2006). Ephemeral streams flow only in direct response to precipitation
including rainstorms, rain on snow events, or snowmelt. They do not receive appreciable
quantities of water from any other source, and their channels are, at all times, above local water
tables. Note that the period over which intermittent flow occurs is a subset of the period of
perennial flow and, similarly, the period of ephemeral flow is a subset of the intermittent flow
period (Fig. SM-1).
The hydrographs shown in Fig. SM-1, although hypothetical, are typical of areas that show
strong seasonality in precipitation patterns; specifically, of areas, such as the Pacific Northwest,
where the majority of precipitation occurs in wet winter periods followed by drier summer
periods. Given the seasonal patterns illustrated in these hydrographs (Fig. SM-1), it can be seen
that any streams that are flowing during the drier summer period are perennial. During the wet
winter period, a flowing stream could be perennial, intermittent, or ephemeral. However, since
ephemeral streams only flow in direct response to precipitation, and because we generally did
not conduct field work within 48 hours of significant precipitation, any stream that is not flowing
during the wet winter must be ephemeral (Fig. SM-1c). Any stream not classified in this way as
perennial or ephemeral was considered intermittent. Note that this methodology could falsely
label some ephemeral streams as intermittent if the stream was flowing during the wet winter in
response to snowmelt. These criteria were used to make initial determinations of streamflow
duration class (initial determinations were sometimes changed based on information from
electrical resistance data loggers; see ‘Actual Streamflow Duration Class’ section of Methods).
To illustrate that the hypothetical hydrographs shown in Fig. SM-1 are typical of the Pacific
Northwest, we provide below brief descriptions of precipitation and discharge patterns in our
five study regions based on an analysis of data for Water Years 2009-2011 from National
Climatic Data Center climate stations and US Geological Survey National Water Information
System hydrology stations (Fig. SM-2 and Table SM-1).
Idaho_c
Central Idaho, as represented by Stanley Ranger Station (Fig. SM-3) has strong seasonal
precipitation, with most precipitation occurring as snowfall. The discharge hydrograph, based on
Squaw Creek, is dominated by snowmelt, as evidenced by the broad seasonal hydrograph; this
1
reflects cold conditions in central Idaho mountains. Once the hydrograph peaks and recedes,
there is very little response to precipitation events (Fig. SM-3), indicating baseflow-fed runoff.
Idaho_n
Northern Idaho precipitation and discharge is also strongly seasonal, as represented by Bayview
Model Basin and EF Pine Creek (Fig. SM-4). The hydrograph is snowmelt-dominated.
However, this region is somewhat warmer than the central mountains of Idaho, so the shape of
the hydrograph (Fig. SM-4) is intermediate between Squaw Creek (Fig. SM-3) and eastern
Oregon streams (Figs. SM-5 and SM-6). As with Idaho_c, the EF Pine Creek shows little
response to precipitation events once the hydrograph recedes, indicating summer baseflow.
OR_e
Precipitation in eastern Oregon, as represented by Barnes Station and Elgin (Figs. SM-5 and SM6) is seasonal with most precipitation occurring as snow in the winter. The Donner und Blitzen
River, located in the permeable mountains of eastern Oregon, has discharge dominated by
snowmelt, as evidenced by a broad seasonal hydrograph (Fig. SM-5). However, short-term
stormflow is evident in response to large rainstorms and rain on snow events. Summer baseflow
is sustained by deep groundwater. The Umatilla River, located in the Blue Mountains of
northeastern Oregon, is very similar to the Donner und Blitzen River (Figs. SM-5 and SM-6).
OR_w
Western Oregon has very distinct wet (fall through spring) and dry (summer) seasons, as typified
by Blodgett (Fig. SM-7). Virtually all precipitation in this region falls as rain, except at upper
elevations in the Cascades. The Siletz River is typical of streams and rivers in western Oregon
mountains (Coast Range and Western Cascades). Streams rapidly respond to rainstorm events,
and there is limited baseflow.
WA_w
Precipitation in western Washington, as represented by Olalla and Olympia (Figs. SM-8 and SM9) has strong seasonality and is dominated by rainfall. Like western Oregon, discharge at Huge
Creek is dominated by short-term stormflow associated with large rainstorm events (Fig. SM-8);
summer baseflow is very low. The hydrograph for the Chehalis River (Fig. SM-9) is very similar
to all western Oregon and Washington streams and rivers. Hydrographs do not drop as quickly
after storms as other example streams, but the patterns still support the three criteria approach to
estimated streamflow duration.
2
References
Leibowitz SG, Wigington PJ, Jr, Rains MC, Downing DM (2008) Non-navigable streams and
adjacent wetlands: addressing science needs following the Supreme Court's Rapanos
decision. Frontiers in Ecology and the Environment 6 (7):364-371. doi:10.1890/070068
Mosley MP, McKerchar AI (1993) Streamflow. In: Maidment D (ed) Handbook of Hydrology.
McGraw-Hill, USA, pp 8.1-8.39
Rains MC, Fogg GE, Harter T, Dahlgren RA, Williamson RJ (2006) The role of perched aquifers
in hydrological connectivity and biogeochemical processes in vernal pool landscapes,
Central Valley, California. Hydrological Processes 20 (5):1157-1175
Rains MC, Mount JF (2002) Origin of shallow ground water in an alluvial aquifer as determined
by isotopic and chemical procedures. Ground Water 40 (5):552-563
Winter TC (2007) The role of ground water in generating streamflow in headwater areas and in
maintaining base flow. Journal of the American Water Resources Association 43 (1):15-25
Winter TC, Harvey JW, Franke OL, Alley WM (1998) Ground water and surface water: a single
resource. Circular 1139, U.S. Geological Survey, Denver, CO
3
Fig. SM-1. Hypothetical hydrographs illustrating maximum duration of flow (Dmax,q) for (a)
perennial, (b) intermittent, and (c) ephemeral streams. Dmax,q is the maximum duration (in
days) of continuous stream or hyporheic flow. Source: Leibowitz et al. (2008).
4
5
Fig. SM-2. Locations of precipitation and discharge stations.
6
Fig. SM-3. Precipitation and discharge at Stanley Ranger Station and Squaw Creek (Region ID_c), respectively,
for Water Years 2009-2011.
7
Fig. SM-4. Precipitation and discharge at Baywiew Model Basin and EF Pine Creek (Region ID_n), respectively,
for Water Years 2009-2011.
8
Fig. SM-5. Precipitation and discharge at Barnes Station and Donner und Blitzen River (Region OR_e),
respectively, for Water Years 2009-2011.
9
Fig. SM-6. Precipitation and discharge at Elgin and Umatilla River (Region OR_e), respectively, for Water Years
2009-2011.
10
Fig. SM-7. Precipitation and discharge at Blodgett and Siletz River (Region OR_w), respectively, for Water
Years 2009-2011.
11
Fig. SM-8. Precipitation and discharge at Olalla and Huge Creek (Region WA_w), respectively, for Water Years
2009-2011.
12
Fig. SM-9. Precipitation and discharge at Olympia and Chehalis River (Region WA_w), respectively, for Water
Years 2009-2011.
Table SM-1. Precipitation and discharge station information. Precipitation stations are from
National Climatic Data Center (http://www.ncdc.noaa.gov/cdo-web/datatools/findstation) and
discharge stations are from US Geological Survey National Water Information System
(http://waterdata.usgs.gov/nwis/uv). Regions: OR_E=eastern Oregon; OR_W=western Oregon;
WA_w=western Washington; ID_n=northern Idaho; ID_c=central Idaho
Region
Station Name
Station ID
Type1
Download
Date
Percent
Complete2
ID_c
STANLEY RANGER STATION
ID US
USW00004112
P
1/23/15
99.5
ID_c
SQUAW CREEK BL BRUNO
CREEK NR CLAYTON ID
13297355
D
1/20/15
100.0
ID_n
BAYVIEW MODEL BASIN ID
US
USC00100667
P
1/16/15
99.2
ID_n
EF PINE CREEK ABV NABOB
CR NEAR PINEHURST ID
12413370
D
1/20/15
100.0
OR_e
BARNES STATION OR US
USC00350501
P
1/20/15
82.6
OR_e
DONNER UND BLITZEN RIVER
NEAR FRENCHGLEN, OR
10396000
D
1/16/15
100.0
OR_e
ELGIN 9.8 N OR US
US1ORUN0001
P
1/20/15
97.1
OR_e
UMATILLA RIVER ABOVE
MEACHAM CREEK, NEAR
GIBBON, OR
14020000
D
1/19/15
100.0
OR_w
BLODGETT 1 N OR US
US1ORBN0013
P
1/16/15
98.0
OR_w
SILETZ RIVER AT SILETZ, OR
14305500
D
1/16/15
100.0
WA_w
OLALLA 1.4 WNW WA US
US1WAKP0013
P
1/20/15
99.0
WA_w
HUGE CREEK NEAR WAUNA,
WA
12073500
D
1/19/15
100.0
WA_w
OLYMPIA 6.5 SW WA US
US1WATH0008
P
1/20/15
97.2
WA_w
CHEHALIS RIVER NEAR DOTY, 12020000
WA
D
1/19/15
100.0
1
P=precipitation, D=discharge
2
Percent of water years 2009-2011 for which data exist
13