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I ,I I " , I I ,I I I ,~ i 1 I I I I I I 2. FIELD DATA COLLECTION The 1995 field work at the Cleopatras Couch and Mathers Hole squawfish spawning sites was conducted under low flow conditions (about 500 cfs) between August 22 and 25, 1995. The work built on an existing data base that has been accumulated at the two sites. Cross sections were originally surveyed at Cleopatras Couch by the U,S. Fish and Wildlife Service (USFWS) in 1983 prior to the flood of record in 1984 (32,300 cfs). The cross sections were resurveyed in 1984 and 1991 (USFWS), and by CRWCO in 1993. Sediment samples were collected in 1991 and 1993 by CRWCO using the Wolman Count technique (Wolman, 1954) or bulk sampling, as appropriate. Cross section surveys and detailed topographic mapping of the spawning bar at Mathers Hole were originally conducted in 1993 (CRWCO). 2.1. Mathers Hole (RM 17.5) At Mathers Hole, twelve of the original fourteen cross sections (XS) were resurveyed (XS1-XS12) and five Wolman Counts (WC) were conducted (Figure 2.1), Cross Sections 13 and 14 which are located in the pool upstream of the spawning bar were not resurveyed. The discharge at the time of the surveys (514 cfs) was determined by gaging of the river at Cross Section 12. The 1995 peak discharge did not inundate the entire bar; the high-water marks were located during the survey. Comparison of the cross section plots (Appendix A) indicates that there was very little change in the bar and channel topography between 1993 and 1995, However, comparison of the 1995 grain size data (Figure 2.2) with the data from 1991 and 1993 indicates that there have been changes in the sediments that comprise the bars (Table 2.1). The data indicate that the median grain sizes (050) of the , two tertiary bars decreased when compared to the 1993 values, In contrast, between 1991 and 1993, the 050 values increased, Comparison of the 1993 and 1995 sediment sizes at the head of the primary bar and downstream of the chute channel just downstream of the left end of XS5 also indicates a fining trend. The 1995 runoff season was characterized by a moderate peak discharge (18,300 cfs) , but a relatively long duration of flows greater than 10,000 cfs, the discharge that Andrews (1978) considered to be the effective discharge for the Yampa River, Since the topographic data at the spawning bar showed very little change between the 1993 and 1995 surveys, it appears that the bar was maintained by delivery of sediment from upstream that is somewhat finer than that which previously covered the bar surface. Incipient motion calculations for XS3 (Mussetter and Harvey, 1994) indicate that in a reach of the river unaffected by backwater, the bed material (050 =84 mm) is mobilized (r.' >1) at discharges between 5,200 and 7,400 cfs (Figure 2.3). Significant sediment transport (r.' >2) does not occur until the discharge reaches about 9,000 cfs. In contrast, at XS14 which is located in the upstream pool, incipient conditions for the same 050 occur at discharges between about 17,000 cfs and 26,000 cfs but significant sediment transport (r.' >2) does not occur until higher discharges (Figure 2.4). The sediments deposited at the spawning bar were, therefore, most likely to have been derived from 2.1 Mussetter Engineering. Inc,