ELECTRONIC SUPPLEMENTARY INFORMATION
Table ES1. Fingerprinting data from a literature review of 13 studies that identified
floodplains/banks as one of several distinct sources. To generate Fig. 1 these data were
sorted within each row from lowest to highest and dots were placed on each line to
indicate the rank of the floodplain/bank among each of the sources.
Table ES2. Beryllium-10 concentrations measured in end-member source areas in
multiple locations.
Table ES 3. Beryllium-10 concentrations measured in channel alluvium and
bank/floodplain deposits. All samples were sieved to < 125 um.
Table ES 4. Lead-210 and Cesium-137 activities measured from suspended sediment
samples collected at gaging stations (UM 1-5 and LM 1-5) and floodplain samples
composite from six locations.
Table ES 5. Beryllium-10 concentrations measured from suspended sediment samples
collected at gaging stations (UM 1-5 and LM 1-5). The 10Be results from UM 4 suggest a
lower percentage of sediment derived from upland sources than 210Pb or 137Cs results,
which is likely caused by a sample processing error (loss of 10Be during leaching) and is
therefore excluded from Figure 7.
Equation 1 and Figure 2 of the main text represent the theoretical expression of all
possible relationships between sediment storage, transport and exchange with the
floodplain for a simple, equilibrium, steady-state system. For tracers such as 10Be with
very long half-lives the percent from each source is:
TSSBe = (%UTotal* BeUP) + (%Bluff * Bebluff )+ (%FP * BeFP)
(ES1)
where TSSBe is the activity of 10Be on a suspended sediment sample, BeUP is the reference
fingerprint of 10Be on upland field sediment, %Bluff is the contribution from bluffs,
Bebluff is the 10Be fingerprint of bluff sediment, %FP is fraction of sediment contributed
from banks or floodplains and BeFP is the fingerprint of 10Be on floodplain sediments.
%UTotal is the total relative contribution from upland field erosion. With respect to 10Be
this includes sediments that have been recently eroded from uplands or eroded upland
sediments that have participated in long-term floodplain storage before being re-eroded
to become TSS.
Assuming direct atmospheric contribution of 10Be to floodplain sediments is minimal,
recently eroded bluff sediments are indistinguishable from eroded bluff sediments that
participated in floodplain storage before becoming part of TSS; and Equation DR1
simplifies to express the total long-term contribution from uplands as:
U10Be = [TSSBe - (%NU * Bebluff)]/ BeUP
(ES2)
Where %NU is the relative amount of sediment eroded from non-upland sources, which
include recently eroded bluff material and eroded bluff sediment that participated in
floodplain storage. For short-lived tracers such as 137Cs and 210Pb, when residence time
in the floodplain is much longer than the half-life of the radioisotope, the contribution
from uplands is simply:
US = [TSSCs - (%Bluff * Csbluff)]/ CsUP
(ES3)
where TSSCs is the activity of Cs or Pb on suspended sediment, %Bluff is the
contribution from bluffs or non-uplands, Csbluff is the Cs or Pb activity on bluff sediments
(treated as zero), and CsUP is the fingerprint of Cs or Pb on recently eroded upland field
sediments. US is the percent of sediment in transport that has been recently eroded from
uplands—upland sediment that has not appreciably participated in floodplain storage.
The amount of sediment eroded from the uplands that has participated in floodplain
storage %Ufs is then simply Equation DR3 minus Equation DR2 or:
ΔU = U10Be - US
(ES4)
For several reasons it is not possible to determine the amount of sediment eroded from
bluffs that has participated in floodplain storage; 1) the half-life of 10Be is longer than the
residence time of sediment storage in the floodplain, 2) atmospheric additions of 10Be to
floodplain sediments is small relative to the amounts on upland sediments and 3) bluffs
have negligible activity of Cs and Pb.
By multiplying the relative contributions described above by the total sediment flux over
a particular period, this method can be used to estimate three important sediment loads:
US: Recent upland load; the sediment recently eroded from upland fields that has not
participated in any significant floodplain/channel network storage.
NU: Bluff or non-upland sediment load; the total amount of sediment eroded from bluffs
or non-upland sources. Sediment may or may not have participated in floodplain storage.
ΔU: Aged upland sediment load; sediment that eroded from uplands at some time in the
past and has experienced a long residence time in channel-floodplain storage.